Among the many open research questions on MTBE are how it is metabolized, ______the levels of exposure in the general population are.

A．A professor from Beijing.
B．An officer of British Airways.
C．A professor in Oxford.
D．A professor from a city near Beijing.

A．Their English are better than native speakers.
B．The contents of the debates are always wonderful.
C．They will have greater achievement in the future.
D．They will become members of British airline companies.