单项选择题

PM2.5 particles are thought particularly damaging to health because they can ______ deep into the lungs.

A．dive
B．penetrate
C．elevate
D．dig
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单项选择题
These researchers intend to ______ this research until there is some substantial finding.
A．insist on
B．call on
C．live on
D．carry on
单项选择题
Light levels are carefully controlled to fall within an acceptable level for ______ reading convenience.
A．intentional
B．ideal
C．obligatory
D．standard
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