题干
My mother can't stop _______when she watches the funny soap opera..
A：laugh
B：laughs
C：laughing
D：to laugh
答案(点此获取答案解析）
C
My mother can't stop _______when she watches the funny soap opera..
A：laugh
B：laughs
C：laughing
D：to laugh
是“甲降落在指定范围”， 
是“乙降落在指定范围”，则命题“至少有一位学员降落在指定范围”可表示为（ ）