top= 0，bott = 10，第五个元素的下标是4

mid = (top + bott)/2 = (0 + 10) = 5(不是，比较1次)，bott = mid - 1 = 4

mid = (top + bott)/2 = (0 + 4/2) = 2(不是，比较2次)，top = mid + 1 = 3

mid = (top + bott)/2 = (3 + 4)/2 =3(整形数运算，有舍弃)(不是，比较次数3)，top = mid + 1 = 4

mid = (top + bott)/2 = (4 + 4)/2 = 4(是，比较次数4)

故应该选择b。