—What is your brother doing?

—He is _____ what to do this summer vacation.[ ]

A. looking after    

B. helping with

C. thinking about  

D. waiting for

焊接三通由高压无缝钢管焊制而成，其接口连接形式可加工成( )等接口形式。

A．焊接坡口

B．透镜垫密封法兰接口

C．平垫密封法兰接口

D．焊接三通