Sentence Correction

Hello,

I just download the flash cards of BeattheGmat.

I have a doubt regarding one of the flash cards:

Beeswax candles burn more cleanly than synthetic candles.

I thought that when you are using a verb in the comparison you must use the "do" before or after the subjet being compared.

for example
Beeswax candles burn more cleanly than synthetic candles do??

what phrase is ok?

Thanks!

While I might even prefer it this way, "Beeswax candles burn more cleanly than do synthetic candles," I feel that the do is necessary and the flash card needs to be fixed.