_______cloning human embryos is not legal in many countries, some scientists are already pushing ahead with research in order to produce a cloned human baby.

________ happens in the world makes us happy and sad by turns. 

It's important to learn how the whole system works. Sometimes, ________， it's the details that interest us the most.

 ________ you have to do the job, you should try your best to do it well even though it may be very difficult for you.

He always brings me a pretty gift ____ he comes to visit me.