comma usage

[Stephanie Pelonia]

which is correct?
a) Your grades are as impressive as they were last semester, if not more so.
b) Your grades are as impressive, if not more so than they were last semester.
c) Your grades are more impressive.

thanks!

[emsr2d2]

Which is correct?
a) Your grades are as impressive as they were last semester, if not more so.
b) Your grades are as impressive, if not more so than they were last semester.
c) Your grades are more impressive.

Thanks!

Which one do you think is natural?
Two are grammatically correct and one is not, but the two which are correct do not mean the same thing as each other.