With Halloween approaching, there exists a real concern among parents and teachers__________ trick or treating will become a serious anti-social activity.
a whether b that
答案是b,既然是concern关注,语气应该是不肯定的吧,求解答
_____________ on the blood of humans and animals ,female mosquitoes often carry a dangerous diease which can spread quickly.
答案feeding,求解释,怎么判断主被动,这里feed的用法是什么