Look! Some people ____ photos on the beach.
A. took
B. takes
C. are taking
D. is taking
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The child ____ on the chair and eating a cake.
A. sitting
B. siting
C. is siting
D. is sitting
Please be quiet! Grandpa ______.
A. sleeps
B. is sleeping
C. sleeping
D. slept
Tom and I ____ standing on the Great Wall of China.
A. am
B. is
C. are
D. be
对于制冷装置的制冷系数描述正确的是：
A. 恒为正
B. 可正可负
C. 0<制冷系数<1
D. 制冷系数>1
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