Questions 19-22

　　Along a street that is currently without trees， seven trees are to be planted in the pattern

　　1 3 5 7

　　2 4 6

　　Where each number designates the position of a tree. No more than two kinds of trees can be planted. If maples are used， no maple can be planted adjacent to or immediately diagonally opposite another maple. Two trees are adjacent to each other if the numbers of their positions differ by two，and immediately diagonally opposite if their numbers differ by one. The following trees，of three kinds，reavailable for planting：Three red oaks Four maples Four sycamores

　　19.Which of the following can be the trees planted along the side of the street that has four trees， in order of their positions beginning with position 1?

　　(A)Maple，sycamore，maple，sycamore

　　(B)Maple，sycamore，red oak，maple

　　(C)Red oak，maple，maple，red oak

　　(D)Sycamore，sycamore，maple，maple

　　(E)Sycamore，sycamore，red oak，red oak

　　20.If red oaks are used，then which of the following must be true?

　　(A)The other trees used are all maples.

　　(B)The other trees used are all sycamores.

　　(C)The red oaks are in positions 1，2，and 3.

　　(D)The red oaks are in positions 3，4，and 5.

　　(E)The red oaks are in positions 4，5，and 6.