If f(x) is continuous on [1,4] and f(1) = 10, f(4)=3 and k =15, then which statement(s) would be true regarding a c value in [1,4] where f(c)=15?
a. a c is not guaranteed to exist.
b. at least one c is guaranteed to exist.
c. there is one c value.
d. there are no c values.
e. there could be one c value.
f. there could be multiple c values.

(could be more than one answer)