问题
选择题
Don't always make your little sister _____. You should be kind to her. [ ]
A. laugh
B. to laugh
C. cry
D. to cry
答案
答案：C
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Don't always make your little sister _____. You should be kind to her. [ ]
A. laugh
B. to laugh
C. cry
D. to cry
答案：C
时方程两根互为相反数
时方程有实数根