Several weeks had gone byI realized the painting was missing.A. asB. beforeC. sinceD.

问题补充：

Several weeks had gone byI realized the painting was missing.

A. as

B. before

C. since

D. when

答案：

A. as

(答案→)B. before

C. since

D. when

解析：本题考查连词的用法。时间had passed(gone)by before S + did…在不知不觉中过……（已过去了）。