Sentence Correction

Varieties of influenza range in severity from mild flus, treatable with bed rest and lasting only a few days, to acute forms, such as bacterial pneumonia, that are potentially fatal.



mild flus, treatable with bed rest and lasting only a few days, to



mild flus, treatable with bed rest and last only a few days, to



mild flus, treatable with bed rest, and they last only a few days, to



those mild flus, able to be treated with bed rest and in duration only a few days, to those



those mild flus which are treatable with bed rest and which last only a few days to those


its a parallelism question..why a is correct and b is incorrect
thanks..