BythetimeDavereachedthesmallboatathickwhitefogoverthesurfaceofthewater.A.spreadB.hasspreadC.hadspreadD.wasspreading
By the time Dave reached the small boat a thick white fog _______ over the surface of the water.
A. spread B. has spread
C. had spread D. was spreading
分 析：
考查时态。句意：by the time...意为“到......的时候”，常与完成时连用，排除A、D；当by the time后跟一般过去时的时候，主句用过去完成时，故选C。
考点：
考查时态
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