Hello I have this apex class and I tried to write a good test class. I obtain a 78% code coverage but I want to know what is missing and what I  have to do to improve it and to improve my skills on test class.

apex class:

public class SearchWithWrapperC {
    
    public  Date StartDate {get;set;}
    public  Date EndDate {get;set;}
  
    
    public List<WrapperClass> listBD {get;set;}
   
    
    public void loadData() {
        listBD = new List<WrapperClass>();
        for(Billing_Detail__c  cr : [Select Id,SF_Opportunity_Id__c,Account_Name__c,Billing_Detail__c, Name,Billing_Type__c, Billing_Period__c , Monthly_Forecast__c,End_of_Billing__c, Amount__c, Billing_Status__c   from Billing_Detail__c Where Billing_Status__c='Authorized for Billing'AND Monthly_Forecast__c>=:StartDate AND Monthly_Forecast__c <= :EndDate  Order by Account_Name__c, Monthly_Forecast__c]){
            listBD.add(new WrapperClass (cr, false));
        }
    }
    
        public List<WrapperClass> getBilling() {
            if(listBD == null) {

        listBD = new List<WrapperClass>(); 
        for(Billing_Detail__c  cr : [Select Id,SF_Opportunity_Id__c,Account_Name__c,Billing_Detail__c, Name,Billing_Type__c, Billing_Period__c , Monthly_Forecast__c,End_of_Billing__c, Amount__c, Billing_Status__c   from Billing_Detail__c Where Billing_Status__c='Authorized for Billing'AND Monthly_Forecast__c>=:StartDate AND Monthly_Forecast__c <= :EndDate  Order by Monthly_Forecast__c ]){
            listBD.add(new WrapperClass (cr, false));
        }
        }       
         return listBD;
        }
    
        
   public PageReference processSelected() {
       Integer m = Date.Today().Month();
       List<Billing_Detail__c> selectedBD = new List<Billing_Detail__c>();
       for(WrapperClass bd: getBilling()){
           if(bd.check_box== true){
               selectedBD.add(bd.cs);
           } 
       }
        for(Billing_Detail__c cs: selectedBD) {
         cs.Monthly_Forecast__c = Date.newinstance(  cs.Monthly_Forecast__c.year() , m ,  cs.Monthly_Forecast__c.day() );
          update selectedBD;
            System.debug('The value is: ' + cs.Monthly_Forecast__c );   
       }
       return null;     
   }
    

    public class WrapperClass {
        public Billing_Detail__c  cs {get; set;}
        public Boolean check_box {get; set;}
        
        public WrapperClass(Billing_Detail__c  c, Boolean check_box){
            this.cs = c;
            this.check_box = false;
        }
    }
}

Test class:

@isTest private class SearchWithWrapperCTest {
@isTest static void testController() {
    Test.startTest();
    		// create the object in input of the class 
        Account a= new Account();
        a.name='Account test t';
        a.RecordTypeId='012w0000000iROM';
        a.Branch__c='Italy';
        a.Status__c='Customer';
        a.BillingCountry='Italy';
        a.Public_Administration_Account__c='No';
        insert a;

		Opportunity testOpportunity = new Opportunity(
		AccountID = a.Id,
		Name = 'Test Opportunity',
		StageName = 'Needs Analysis',
		CloseDate = date.today().addDays(15)        

		);
		insert testOpportunity;
    
    
    
    Billing_Detail__c c= new Billing_Detail__c(); 
		c.Amount__c= 100;
		c.Billing_Type__c='FEE';
		c.Billing_Period__c='monthly';
		c.Billing_Status__c='Authorized for Billing'; 
		c.Product_Line__c='Interactive Experience (iX)'; 
		c.Product__c='Chat Delivery'; 
		c.Monthly_Forecast__c=Date.newInstance(2018, 7, 9); 
		c.End_of_Billing__c=Date.newInstance(2019, 7, 9); 
		c.Billing_Detail__c=testOpportunity.Id;

		insert c;
		Date StartDate= Date.newInstance( 2018, 10,1);
    	Date EndDate= Date.newInstance(  2018,10,18 );
    
    list<WrapperClassTest> wrp= new  list<WrapperClassTest>();
    	ApexPages.StandardController sc = new ApexPages.StandardController(c);
		 SearchWithWrapperC x =new  SearchWithWrapperC();
    	x.loadData();
    	x.getBilling();
    for(WrapperClassTest wTest :wrp)
  {
   wTest.check_box=true;    
  }
    	x.processSelected();  
    
	
	System.assert(wrp.size() > 1);
     Test.stopTest();
}
    Public class WrapperClassTest{
        public Billing_Detail__c  cs {get;set;}
        public Boolean check_box {get;set;}
        public WrapperClassTest(Billing_Detail__c  c, Boolean check_box)
        {
            this.cs = c;
            this.check_box = false;
        }
    }
}

Thanks