Learning a foreign language calls for your memory,time,patience and emotions.________ it is not a simple thing.
I have seen trees, _____ open at sunrise and close at sunset.
___ is known to all, China is a developing country, belonging to the third world.
She showed her new watch to my wife, ___ was kept in a box.
The reason ______he didn't come was ______he was ill.
We travelled together as far as Chicago, _______ we parted.