The meeting had to be ____ because of the absence of chairman of the board.
A. called out
B. called off
C. called on
D. called up
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Bicycling is good exercise;______, it doesn't pollute the air.
A. however
B. moreover
C. therefore
D. otherwise
真核生物mRNA生物合成的叙述正确的是
A. 5'端加帽，3'端加poly A尾
B. 与蛋白结合形成核糖体
C. 剪除外显子，连接内含子
D. 3'端加CCA序列
E. 三叶草型结构
有关tRNA的叙述错误的是
A. 是RNA聚合酶Ⅲ的转录产物
B. 3′端的CCA-OH是经过加工生成的
C. 分子中含有大量稀有碱基
D. 分子量最小的一类RNA
E. 分子中含有密码子
RNA聚合酶核心酶的组成是
A. α β β′
B. αβ β′ σ
C. α2 β2
D. α2 β β′ω
E. α2 β β′ σ
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