求各位大神指出错误
Dim sum, a, b, c, d, f As Singlea = TextBox2.Text
b = TextBox3.Text
c = TextBox4.Text
d = TextBox5.Text
f = TextBox6.Text
sum = a + b + c + d + f
If sum > 450 Then
MsgBox("学生" + TextBox1.Text + "的成绩为优秀!!")
Else If a >=88,b>=88,c>=88,b>=88,c>=88,d>=88,f>=88 then
MsgBox("学生" + TextBox1.Text + "的成绩为优秀!!")
Else If a >=95,b>=95,c>=95 and d>=80,f>=80 then
MsgBox("学生" + TextBox1.Text + "的成绩为优秀!!")
Else
MsgBox("学生" + TextBox1.Text + "的成绩不是优秀!!")
End If
这个选择结构错在哪里啊