题干
Someone called me up at midnight, but he had hung up I could answer the phone.
A：as
B：since
C：until
D：before
答案(点此获取答案解析）
D
时，要使其中一个未知数的系数相等或互为相反数，必然适当变形，以下四种变形中正确的是（ ）
②
③
④