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There are many problems with travelling through time. The commonest are the so-called paradoxes.
Yeah, like what happens if you meet your grandfather? (It doesn't matter which side of the family.) What if you should happen to kill him before he met your grandmother? That would mean that you had never been born. That would mean, of course, that you couldn't have traveled back in time and you couldn't have killed your grandfather. Then, of course, you were born after all.
In the following context, which verbs is or are is best?
Although 'common' does in fact take '-er' in the comparative (commoner) and '-est' in the superlative (commonest), in my dialect of English, 'most common' sounds more natural:
The most common problem is the paradox.
(singular + singular + singular)
The most common problems areparadoxes.
(plural + plural + plural)
the commonest "is" or "are" the so-calle 
Linear Mode
