After the Asian Games, ______ people came to Guangzhou for a visit during holidays. [ ]

A. thousand

B. thousands of

C. five thousands

D. five thousands of

换热设备按（）分为表面式换热器、蓄热式换热器、液体间接式换热器、直接接触式换热器。

A.用途

B.热量的授受方式

C.结构形式

D.材料