哺乳动物受精过程有哪些机制阻止了多精子与卵细胞受精？（）①精子获能具有差异性②精子穿
题型：单项选择题
问题：
哺乳动物受精过程有哪些机制阻止了多精子与卵细胞受精？（）
①精子获能具有差异性
②精子穿越透明带触及卵黄膜，导致透明带反应
③精子触及卵黄膜表面微绒毛的合抱反应
④精子入乱后，触发卵黄膜封闭作用
A.①②
B.②③
C.③④
D.②④
题型：单项选择题
阅读下面短文，按照句子结构的语法性和上下文连续的要求，在空格处填入一个适当的词或使用括号中词语的正确形式填空，并将答案填写在答题卷标号为的相应位置上。
Roadside bomb explosions in southern Afghanistan have killed at least five civilians and five policemen. The attack came as NATO’s new Secretary General visited Kabul 小题1: (discuss) with Afghan officials and candidates ahead of the August 20 小题2: (president) election.
Afghan officials in southern Helmans province said a group of civilians小题3: (travel) to a wedding party when 小题4: big vehicle, which was being pulled by a tractor, hit a roadside bomb.
Officials have blamed Taliban insurgents（叛乱分子）as part of efforts to discourage voters 小题5: taking part in the coming election.
Five policemen were also killed in a separate roadside bomb burst in Helmand, 小题6:region is a known Taliban
stronghold（据点） 小题7:thousands of US Marines and British soldiers are engaged 小题8:major anti-insurgency operations to improve security小题9:Afghans go to vote.
The security situation deteriorating（恶化）in southern parts of the country has been a cause of concern for both Afghan election officials and international observers since the 小题10:(remove) of the Taliban from power in 2001.