Mother told Jim to watch the milk until it boiled and then _________ off the gas.

A．turn

B．turning

C．turned

D．having turned

以下语句中那句可以选择指定的表项或取消已选择的表项

A．列表框.Selected(索引值)

B．列表框．Selected(索引值)；True|False

C．列表框.Selected

D．列表框.Selected=True |False