题目

Among the biggest problems facing those remote mountainous villages __________the lack of experienced teachers. A . is B . are C . is there D . are there 答案：A下列属于社会行为的是A．海葵和寄居蟹在一起生活B．在一片草地上生活的许多昆虫C．一只蚜虫被天敌咬伤后，能释放信息激素，使同种其他个体尽快逃避D．生活在草原上的草、昆虫、青蛙、蛇和鹰等