It's dangerous to swim _____ at sea.这里应该填什么介词或副词?

It's dangerous to swim _____ at sea.这里应该填什么介词或副词? It's very dangerous to swim in a d____ river.中应该填什么? 1.It's too dangerous to swim in the sea. 2.I have two studie I can't use my office at present,it's ____.填一个介词或副词. 用适当的介词或副词填空.It's easy for me to swim ____ the river. Don't___(go)to high It's a dangerous._____处填什么? It's dangerous to swim here.look at the sigh.OH ,I ________n It is very cold_填什么介词或副词 it's,swim,dangerous,to,alone(.) 连词成句 it's,swim,dangerous,to,alone 连词成句 It's very dangerous to swim at the pool 解释下为什么用to?是固定的吗? 用适当的介词或副词填空.This mustn't jump ___ the tree.It's dangerous.