It is _______ he often fails in exam_______ makes his parents worried about him. [      ]

A. because, that      

B. that, that        

C. 不填,  that    

D. what, that

根据卷烟制丝生产过程中各设备的特点和功能，将卷烟制丝设备分为加湿设备、（）、切丝设备、干燥设备和其它设备五类。