i have created this vf page. How can I create a hyperlink so that when they click on subject, it takes them to the event record

<apex:page standardController="contact"   extensions="ActivityController">
<apex:pageBlock title="Event History" >
<apex:pageBlockTable value="{!Activities}" var="a">
<apex:column value="{!a.activitydate}" />
<apex:column value="{!a.subject}"/>
<apex:column value="{!a.whoid}"/>
<apex:column value="{!a.whatid}"/>
<apex:column value="{!a.type}"/>
</apex:pageBlockTable>
</apex:pageBlock>
</apex:page>