请指出it在下列各句中的用法.
may i use your dictionary?sorry,i left it at home.
_________________________________________
it is a good idea to bring a camera when we travel around.
__________________________________________
it is polite knocking on the door begor entering a room.
__________________________________________
i found it difficult to protect the environment by oneself.
___________________________________________
it's said that MH370 has been discovered in the Indian Ocean.
____________________________________________