What the science teacher does and says _________ of great importance to college students. A.was B. are C. is D. were 答案:C 解析: 主语从句作主语,谓动作单数;does and says 提示时态应为一般现在时。已知,动点P满足|PF1|+|PF2|=4,记动点P的轨迹为E.(1)求E的方程;(2)曲线E的一条切线为l,过F1,F2作l的垂线,垂足分别为M,N,求|F1M|•|F2N|的值;(3)曲线E的一条切线为l,与x轴分别交于A,B两点,求|AB|的最小值,并求此时切线的斜率.