单项选择题

The cells contribute to immune defenses in two major ways: some direct and ______immune responses whereas others directly attack infected or cancerous cells.

A．regulate
B．activate
C．retain
D．consolidate
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单项选择题
Veteran doctors can often reflect on past experiences to draw lessons for the present; young resident doctors, however, rarely have the advantages of such______.
A．vagueness
B．misconceptions
C．retrospection
D．hostility
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