It seems that manufacturers will remain reluctant to make improvements unless ______.
A. they can do so at a very Iow cost
B. they find their machines hard to operate
C. they have difficulty selling their products
D. they receive a lot of complaints about their machines
肿瘤见于（）
A. T淋巴细胞缺陷
B淋巴细胞缺陷
C. 巨噬细胞缺陷
D. 中性细胞缺乏
E. 自然杀伤细胞缺陷
白细胞黏附功能缺陷（）
A. 低钙抽搐
B. 新生儿脐带延迟脱落
C. 口服脊髓灰质炎活疫苗可引起患儿肢体瘫痪
D. 扁桃体变小或缺如
E. 可自然痊愈
The author had trouble operating his VCR became ______.
A. he had neglected the importance of using the timer
B. the machine had far more technical features than necessary
C. he had set about using it without proper training
D. its operation was far more difficult than the designer intended it to be
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