单项选择题

A．The Rhythmic Cycle of the Sun and the Tide
B．Discoveries in Biology
C．A Scientific Study
D．A Living Clock
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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
填空题
A．whichB．whatC．whoD．whom
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