The maths problem is very easy, so I can work it out ________.
A：in trouble
B：without difficulty
C：by heart
D：with your help
下面四个数中，负数是（ ）
We need to come up with a __________ and make a decision at once
青藏高原夏季气温比同纬度的长江中下游平原低得多，其主要原因是受（ ）
