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【单选题】
试验性研究和观察性研究的最主要区别点在于A.
观察性研究的分组是自然形成，试验性研究系研究者控制的
B.
试验研究的各组样本数相等，观察性研究不相等
C.
试验研究时间短，，观察性研究周期长，难进行
D.
观察性研究偏倚多，试验性研究偏倚少
E.
观察性研究可行性好，试验性研究难进行
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【单选题】— _____ have you been learning English? — I have been learning English for six years. [ ]
A.
How long
B.
How often
C.
How far
【单选题】—Oh, you have a headache and a cough.______ have you been like this? —Two days. [ ]
A.
How many
B.
How long
C.
How often
【单选题】—How many times have you been to China? —In fact, it is the first time I __________ here.
A.
have been
B.
am
C.
had been
D.
was
【单选题】—Oh, you have a headache and a cough. __________ have you been like this? — Two days.
A.
How long
B.
How often
C.
How old
D.
How many
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