Among the theories to explain the origin of AIDS, the polio vaccine theory, in the opinion of the author of the article, seems to be the most plausible one.

A．One.
B．Two.
C．Three.
D．Four.

A．Professor Simms kept the class late.
B．He couldn’t find Professor Simms’s classroom.
C．Professor Simms, lecture is about the sea.
D．He didn’t understand Professor Simms’ lecture.