. The concert was a great success--there wasn’t a single empty seat in the concert hall

问题补充：

. The concert was a great success--there wasn’t a single empty seat in the concert hall, it was very cold.

A. because

B. when

C. unless

D. although

答案：

A. because

B. when

C. unless

(答案→)D. although

解析：本题的含义是这场音乐会很成功，虽然天气很寒冷，但在音乐会大厅没有一个空位，although表示虽然的意思，故本题选D。