Other students joined in the protest, and ________ a large crowd

Other students joined in the protest, and ________ a large crowd gathered outside the building.
A、after all
B、as soon as
C、long before
D、before long
【正确答案】：D
【题目解析】：本题考查介词短语的辨析。before long表示“不久以后，很快”。