单项选择题

According to 1970 figures in the U.S. men smokers are ______.

A．twice as many as women smokers
B．two times as many as women smokers
C．1.5 times as many as women smokers
D．seventeen percent less than women smokers

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单项选择题

One of the major difficulties of research into human health problems is the difficulty of ______.

A．knowing how many factors there are
B．isolating particular, individual factors from the complex human environment
C．knowing the complex human environment
D．increasing the number of the competent physicians

单项选择题

Since 1939, numerous scientific studies ______.

A．proved smoking causes fatal diseases
B．proved smoking causes life-shorting diseases
C．proved smoking is a health risk
D．had been made to prove whether smoking causes life shorting