M: You look worn out. Are you feeling under the weather?
W: Not at all. But I have been putting in some long hours in the chemistry lab.
Q: What does the woman mean?（）

A．their legs and feet have strong skin and muscles
B．their blood vessels are stiff and tough legs
C．they wear a kind of special suit
D．there are lots of one-way valves in the veins of their legs

A．The Constitution, one of the great documents of the world, was written by fifty- five men of varying talents and backgrounds.
B．The Constitution owes its greatness to the fact that it has never changed in a changing world.
C．The Constitution was written by many people working together cooperatively.
D．The Constitution was written by many people, all of whom were thinkers of the highest order.