Declining revenues resulting from a decrease in business travel,a source of income without which most commercial airlines could not survive,are
going to force many commercial airlines to in crease prices and decrease services in the coming months.
a) a source of income without which most of airlines could not survive,are
b) a source of income without which most of airlines could not survive,is
c) and most commercial airlines use it as a source of income to survive with,are
d) which is a source of income which is needed by most commercial airlines who could not survive without it,are
e) which most commercial airlines use as a source of income without which they are unable to survive ,is
the answer given is A
But i think it should be " B" as "Declining revenues.........................................travel" is a phrase which is always singular.
Kindly help