假设现在有两个变量分别是x和y，其中x=3，y=5，以下选项中______不能交换两个变量中的值（）
A. x,y=y,x
B. x=y
C. y=x
D. C．t=x
E. x=y
F. y=t
G. D．x=x+y
H. y=x-y
I. x=x-y
The doctor suggested that the patient（）smoking and drinking
A. cut away
B. cut down on
C. cut back
D. cut off
Which of the following bonds can be treated as weak bonds or secondary bonds（）
A. dipole–dipole interactions
B. the London dispersion force
C. hydrogen bonding
D. metallic bonding
What are the characteristics of the same main group in the periodic table（）
A. The outermost layers have the same number of electron
B. They have the similar chemical properties
C. The number of electron layers from top to bottom gradually increases
D. The atomic radius from top to bottom gradually increases
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