Radio buttons are grouped in a "radiogroup", only one radio button could be checked within the group. For selecting multiple items, use checkboxes.
Update: The radiogroup can be placed anywhere, simply use radiogroup attribute to assign radio's group.

<window>
	<div height="5px" />
	ZK Online Survey Demo
	<radiogroup id="sv1" onCheck="choice1.value = self.selectedItem.label" />
	<radiogroup id="sv2" onCheck="choice2.value = self.selectedItem.label" />
	<radiogroup id="sv3" onCheck="choice3.value = self.selectedItem.label" />
	<grid>
		<rows>
			<row><cell colspan="5">Which support would you like ZK to improve upon?</cell></row>
			<row>
				<radio id="ra1" label="Web IDE Support" radiogroup="sv1" />
				<radio id="ra2" label="Bug Fixing" radiogroup="sv1" />
				<radio id="ra3" label="Performance" radiogroup="sv1" />
				<radio id="ra4" label="Backward Compatibility" radiogroup="sv1" />
				<radio id="ra5" label="Styling" radiogroup="sv1" />
			</row>
			<row><cell colspan="5">Which site you like ZK to improve upon?</cell></row>
			<row>
				<radio label="Offical Web Site" radiogroup="sv2" />
				<radio label="ZK Demo" radiogroup="sv2" />
				<radio label="ZK Sandbox" radiogroup="sv2" />
				<radio label="Forum" radiogroup="sv2" />
				<radio label="On-line Training" radiogroup="sv2" />
			</row>
			<row><cell colspan="5">Which document you have read is best?</cell></row>
			<row>
				<radio label="ZK Essentials" radiogroup="sv3" />
				<radio label="Style Guide" radiogroup="sv3" />
				<radio label="Developer Guide" radiogroup="sv3" />
				<radio label="Developer Reference" radiogroup="sv3" />
				<radio label="Installation Guide" radiogroup="sv3" />
			</row>
		</rows>
	</grid>

	<hlayout>
		Feature :
		<label id="choice1" />
	</hlayout>
	<hlayout>
		Web Site :
		<label id="choice2" />
	</hlayout>
	<hlayout>
		Documentation :
		<label id="choice3" />
	</hlayout>

</window>