____ he opened the window, a sparrow flew in.
A. Though
B. The moment
C. If
D. Unless
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Bad habits, _____ formed, are difficult to give up.
A. and
B. once
C. but
D. or
---May I go now?---________ you've finished your work, you may go.
A. Now that
B. As soon as
C. As long as
D. Although
Never lose heart ______ difficulty you may meet with.
A. what
B. whatever
C. how
D. however
两束完全相同的光线在分别通过两段长度相同、折射率不同的材料后，由于它们：
A. 经过的路程相同，因此相位差相同
B. 经过的光程相同，因此相位差相同
C. 经过的光程不同，因此相位差不同
D. 经过材料的折射率不同，因此相位差不同
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