题目

An increasing number of learners are buying online courses in the belief ________ they are much convenient and cheaper. A . which B . if C . whether D . that 答案：D如图，正方形ABCD的面积为25，△ABE是等边三角形，点E在正方形ABCD内，在对角线AC上有一点P，使PD+PE的和最小，则这个最小值为_____________。