The classroom is big enough for 25 students for normal use you need more space for special

问题补充：

The classroom is big enough for 25 students for normal use you need more space for special activities.

A. once

B. because

C. if

D. unless

答案：

A. once

B. because

C. if

(答案→)D. unless

解析：连词辨析。A. 一旦 B. 因为 C. 如果 D.除非。句意：除非你需要更多的空间做特殊的活动，这间教室对于正常使用，容纳25个学生足够大了。选D。