How to Rerender multiple sections in VisualForce page?

Sample Code:

Visualforce page:

<apex:page controller="sample" sidebar="false" >
<apex:form >
    <apex:pageblock >
        <apex:pageblockSection id="c" title="Contact Details" >
            <apex:pageBlockTable value="{!con}" var="Contact ">
                <apex:column value="{!Contact.Name}"/>
            </apex:pageBlockTable>
        </apex:pageblockSection>
        
        <apex:pageblockSection id="a" title="Account Details" >
            <apex:pageBlockTable value="{!acc}" var="account">
                <apex:column value="{!account.Name}"/>
            </apex:pageBlockTable>        
        </apex:pageblockSection>      
        <apex:pageBlockButtons location="bottom" >
            <apex:commandButton value="Fetch" reRender="c,a" action="{!fetch}"/>
        </apex:pageBlockButtons>  
    </apex:pageblock>
</apex:form>
</apex:page> 

Apex Controller:

public class sample
{
    public List<My_Contact__c> con{get;set;}
    public List<Account> acc {get;set;} 
 
    public void fetch()
    {
        mem = [SELECT Name FROM My_Contact__c];
        acc = [SELECT Name FROM Account];     
    }    
}

Output :

Before Click on fetch :

After click on fetch :