Using infinitives as the object

[farazmit]

Hi,

I was just reading a Standard test question with the following two options:

The best thing to do is leave it alone, let it dry and ....
The best thing to do is (to) leave it alone,.....

I thought the (to) is required in the example above but the answer explanation says it is not. Is that correct ? Is there any rule about when you want to use (to) in this case ?

Thanks,
Faz

[5jj]

It's fine with or without 'to'.