—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
网友回答
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