He found it increasingly difficult to read, ________ his eyesight was beginning to fail.
A. though
B. for
C. but
D. so
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I haven't seen Ann for ________ long that I've forgotten what she looks like.
A. such
B. very
C. so
D. too
He tried his best to solve the problem, ________ difficult it was.
A. however
B. no matter
C. whatever
D. although
位错属于哪一种缺陷
A. 肖特基缺陷
B. 弗伦克尔缺陷
C. 线缺陷
D. 面缺陷
P型半导体
A. 可以由受主杂质P原子掺杂得到
B. 是以空穴作为多数载流子的
C. 在热平衡状态下的空穴浓度近似等于电子浓度
D. 不可以转变为N型半导体
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