初学者Linq的一个问题

Question

代码如下：
DataQueryable countOfUser;
try{
countOfUser = ( from o in wkSpace.Application.Users
where o.FirstDate >= firstDate
where o.LastDate <= lastDate
where o.HasFinshed != true
where o.FinishDate != null
select o);
}
其中，FirstDate、LastDate、FinshDate是date类型，HasFinished是bool类型

现在的问题是，当我把后两句where去掉后，查询结果从有结果，变成没有结果了

Answer 1

where o.FirstDate >= firstDate
where o.LastDate <= lastDate
->
where o.FirstDate >= firstDate && LastDate <= lastDate