_ _ _ _ _ It's such hot weather to go swimming! This is so interesting.

Test analysis: investigate the usage of exclamatory sentences. When followed by a noun or noun phrase, use what to guide the exclamatory sentence; When it is followed by an adjective or adverb, use how to guide the exclamatory sentence. The fun in this sentence is an uncountable noun, so it is used to sigh. What a pleasure it is to go swimming in such hot weather! Therefore, A is correct.

Comments: exclamatory sentences are an important usage in senior high school English. After a noun or noun phrase, use what to guide the exclamatory sentence. When it is followed by an adjective or adverb, use how to guide the exclamatory sentence.