单项选择题

A.val b = 2 * a // b 等于 Array(2,4,6)
B.val b = a.map(_*2) // b 等于 Array(2,4,6)
C.val b = for(elem <- a) yield 2 * elem // b 等于 Array(2,4,6)
D.val b = for(elem <- a if elem % 2 == 0) yield 2 * elem // b 等于 Array(4)