英语单选,求给出解释,括号里是答案

题目

英语单选,求给出解释,括号里是答案
It's been discovered in the study _____  people are more relieved after forgiving others.(that)
It was unusual that such cloth neighbors _____ not know one another.(should)
The agent promised to keep the family members fully ___ of any development.(informed)
I suggest you ____ him some specific questions about  his work experience.(ask) 

答案

1.people are...是宾语从句,做discovered的宾语,故用that.2.unusual等某些形容词后面的从句的谓语要用should+动词原形或直接用动词原形,故这里用should.3.这里是保证家庭成员被完全告知,用被动语态,故是informed,而...