Choose the most appropriate option from the ones given below to complete the following passage. (3)
Scientists studying the morphology of the human race (a) _______ of the opinion that throughout the 20 0,000 years of the history of modern humans we, (b)_______ the shortest, lightest and the least robust. Even our brains are the smallest. Two hypotheses ( c)_________ been put forward to explain the reasons behind these reductions in size.

a.

are
is
were
will be

b.

is
are
has
has been


c
has been
have been
will have been
are
​