Some researchers believe that there is no doubt ______ a cure for AIDS will be found.

A．which

B．that

C．what

D．whether

依据X射线探伤胶片来评定焊缝质量时，I级焊缝表示焊缝内无（）焊接缺陷。

A、裂纹

B、未焊透

C、未熔合

D、气孔