“a=1”是“直线x+y=0和直线x-ay=0互相垂直”的[ ]
A.充分而不必要条件
B.必要而不充分条件
C.充要条件
D.既不充分也不必要条件
答案：C
—Must I stay here with you?
—No , you____.Just do your own business. [ ]
A. mustn't
B.needn't
C.can't
D.may not
坏死性小肠炎（）
A.稀水便伴脐周痛
B.黏液脓血便伴下腹痛
C.血便伴里急后重
D.洗肉水样大便
E.腹泻与便秘交替，常有右下腹痛
