Quoting in asp.net

Hi,
From http://forums.asp.net/post/5158249.aspx, 
What would the difference be for the following two snip:
<asp:SqlDataSource ID="SqlDataSource1" runat="server" ConnectionString="<%$ ConnectionStrings:MyConnectionStrings %>" SelectCommand="<%=String.Format("abc")%>"> </asp:SqlDataSource>

<asp:SqlDataSource ID="SqlDataSource1" runat="server" ConnectionString="<%$ ConnectionStrings:MyConnectionStrings %>" SelectCommand='<%=String.Format("abc")%>'> </asp:SqlDataSource>
what's the difference between single quotes and double quotes?
how to properly quote in quote? I.e., within the above 
SelectCommand='<%=Strin ...