For some birds the sense of smell appears to paly 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 答案是B，我选的A。不明白为什么要用现在完成时，而且感觉困难应该是复数。麻烦老师讲解。