【华慧考博】考博英语每日一练（11.21）考博加油！
1. The spontaneous movement of water across a membrane ______ gradient of water concentration is called osmosis.
A. drives a
B. driven by a
C. has driven
D. drove
2. To explore mechanical scent detection, a search institute has developed a synthetic nose which can detect scents in vapors at a _______ of one particle to a million.
A. number
B. percentage
C. proportion
D. ratio
【考博英语补全句子】
1. She dances ________ half an hour to ________ ________. （为了保持健康，她每天跳舞半小时。）
2. Beef gives them much ________ ________ their after-school ________. （牛肉为他们的课外活动提供许多能量。）
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