It may be three or five years _______ it is possible to test thismedicine on human patients according to scientists.
A. since
B. before
C. after
D. when
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A company cannot operate efficiently _______ it is free from suchinterference.
A. so long as
B. so that
C. unless
D. because
It wasn’t such a good party _______ the host had promised us.
A. that
B. which
C. as
D. what
游离型Down综合征为（ ）染色体数目畸变
A. 单体型
B. 三体型
C. 单倍体
D. 三倍体
E. 多倍体
游离型21三体综合征形成最常见的原因是（）
A. 双亲在减数分裂过程中21号染色体不分离
B. 母亲在第一次减数分裂过程中21号染色体不分离
C. 母亲在第二次减数分裂过程中21号染色体不分离
D. 受精卵卵裂过程中21号染色体不分离
E. 双亲之一是21与D组或G组的罗伯逊易位携带者
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