Tom clean up the broken glass someone walks on it.A. sinceB. whenC. beforeD. as

问题补充：

Tom, clean up the broken glass someone walks on it.

A. since

B. when

C. before

D. as

答案：

A. since

B. when

(答案→)C. before

D. as

解析：考查连词：A.since自从，既然，B. when当…时候，C. before在…前面，D. as 正如，因为，虽然，句意：汤姆，在有人踩在上面之前把破碎的玻璃打扫干净。选C。