I was     the edge of the canyon. A river was      the bottom of the canyon.

A．at；on

B．on；in

C．on；at

D．in；at

— We did not see him at the lecture yesterday．

— You _______ him, since I went to an international conference with him．

A．mustn’t have seen

B．can’t have seen

C．couldn’t see

D．needn’t have seen