L=2: R=6
Do While L = R
If Then
t = a(L): a(L) = a(R): a(R)= t
Else
End If
Loop
t = a(R): a(R) =a(1): a(1)=t
上述程序段中方框处可选语句为:( )
① a(L) > a(1) ② a(L) > a(R) ③R = R - 1 ④L = L + 1