33、So often ______ in recent years that it has almost become a rule.

A. has this happened              B. this has happened

C. is this happening               D. for this to happen

33、A

請(qǐng)?jiān)谶@里輸入關(guān)鍵詞:
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2.III-8的基因型是        。該系譜圖中,屬于她的旁系血親有                  
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