单项选择题

Vaccines for viral diseases are often produced from ______.

A．plant cells
B．human hormones
C．crop plants
D．bacteria
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单项选择题
The author believes that everything in your pack should ______.
A．be lightweight
B．serve an important purpose
C．serve several purpose
D．keep the pack warm
单项选择题
According to the passage, if you were camping in 10℃ weather and the temperature dropped to 10℃ below zero, you could compensate for this by______.
A．sleeping in a tent with special layers of clothing
B．sleeping in a tent on the side of a hill
C．sleeping on the side of a hill
D．sleeping on the side of a hill out of the wind
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