单项选择题

下面程序运行后输出是 ______。
Private Sub Form_Click()
for i=1 to 4
x=1
for j=1 to 3
x=3
for k=1 to 2
x=x+6
next k
next j
next i
print x
End Sub

A.7
B.15
C.157
D.538