How to set default value in <lightning:select> in Salesforce?

<aura:application extends="force:slds" controller="LightningAccountController">
<aura:attribute name="accounts" type="Account[]" />
<aura:handler name="init" value="{!this}" action="{!c.doInit}" />

<lightning:select label="Choose 1" name="a_opt" aura:id="a_opt" onchange="{!c.change}">
<aura:iteration items="{!v.accounts}" var="account">
<option value="{!account.Id}">{!account.Name}</option>
</aura:iteration>
</lightning:select>
</aura:application>