If you want to keep healthy, you must be_______.
A. optimistic.
B. pessimistic
C. discouraged
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From the dialouges of this part, we can learn that it is helpful for our health except___.
A. doing more exercises
B. keeping balanced diet
C. sleeping all day
凸轮机构通常由下列构件组成()
A. 主动件、凸轮和机架
B. 主动件、从动件和推杆
C. 凸轮、推杆和机架
D. 主动件、凸轮和从动件
凸轮机构中,使凸轮与从动件保持接触的方法有: 和 。