—We’ll visit the Summer Palace there is a heavy rain tomorrow. — OK! Boating on the lake m

问题补充：

—We’ll visit the Summer Palace there is a heavy rain tomorrow.

— OK! Boating on the lake must be great fun.

A. if

B. as soon as

C. unless

D. since

答案：

A. if

B. as soon as

(答案→)C. unless

D. since

解析：考察连词。if 如果；as soon as一……就；unless 除非；since自从……以来。句意：我们要去参观颐和园，除非明天有大雨。——好吧，在湖上泛舟肯定很有趣。故选C。

故选C