Mountain climbing ________ be very tirin
题型：选择题
问题：
Mountain climbing ________ be very tiring, and many people feel sick ________ the atmosphere gets thinner.
A．may; while
B．can; as
C．must; though
D．should; for
Mountain climbing ________ be very tiring, and many people feel sick ________ the atmosphere gets thinner.
A．may; while
B．can; as
C．must; though
D．should; for