题目

It is reported that no problems have been discovered in milk products such as Yili, Sanlu and Mengniu, ________ proves newly-produced liquid milk safe.        A. that    B. what C. which       D. where 答案：C下列各一元二次不等式中，解集为空集的有________．①（x+3）（x-1）＞0；②（x+4）（x-1）＜0；　③x2-2x+3＜0；　④2x2-3x-2＞0．