-----What about going to the South Lake for a picnic next weekend?
----OK. But I am not sure ____it will rain.
A. why B. where C. when D. whether
D
【解析】
试题分析：句意：下周末去南湖郊游怎样？好的。但是我不确定是否会下雨。whether为是否。结合句意，故选D
考点：考查连词的用法
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