下列程序共执行了______次循环。
Private Sub Command1_ Click ()
Dim intsum As Integer
Dim i AS Integer
Dim j As Integer
i = 1
j = 1
do
do
intsum = intsum +
j =j+2
Print j;
Loop until j ＞3
i = i+1
Loop until i＞17
Print
Print intsum
End Sub

A．14

B．15

C．14

D．13

导管、管胞、筛管、筛胞分别存在于何类植物体中及存在于哪一部分？各输导什么物质？