The main reasons for the rapid growth of tourism revenue in China

A does not conform to the meaning of the question and does not constitute a causal relationship with the stem of the question;

B conforms to the problem. The fundamental reason for the rapid development of China's tourism industry is the sustained and rapid economic development;

CD does not meet the meaning of the question, which is one of the reasons for the rapid development of tourism in China, but it is not the fundamental reason;

So the answer is B.