Having more than the usual numbers of fingers or toes on the hands or feet is termed polydactyly.

A.

B. Having had more than the usual number of fingers or toes

C. Having more than the usual number of fingers or toes

D. To have more than the usual number of fingers or toes

E. To have more than the usual numbers of fingers or toes

confused between C and D. In fact I have two questions:

1. the actual confusion is when to use gerund and when to use infinitive.

2. the explanation says that "parallelism" is an issue here. could someone please elaborate?

Thanks
na

we are now having a debate about this question. it has been in the cat pool since before i started working for mgmat, so i can't tell you the source of its inspiration (and, more importantly, whether there is an official precedent for the gerund/infinitive distinction being made here).

as a native speaker of english, i will staunchly vouch for 'having', although the exact explanation is elusive. the first few examples of 'infinitive nouns' i can conjure are all <i>abstractions</i> ('to know is to love', 'to become a mother of six was her greatest ambition'), while the first few examples of these gerunds tend to be concrete notions like the one presented here.
i will not go so far as to posit a general principle, at least not as yet.