For some birds the sense of smell appears to play a role in navigation, since pigeons with surgically removed olfactory nerves were found to have increased difficulties in homing.
(A) were found to have increased difficulties
(B) have been found to have increased difficulty
(C) were found to have increasing difficulty
(D) had been found to have increased difficulties
(E) have been found to have increasing difficulties

Official Answer: B

Please explain.

Thanks,
Iwill