It was _____ it raised so many difficult questions _____ the book took a long time to come out.

_____ you can chat with your friends, makes communication easy.

He tried to deal with the ever-increasing burden of his work，but finally _____ and had to take a complete rest.

However frequently 　　　　, the textbook play acted by my classmates still interests us every year.

Many people can’t learn any lessons from themistakes they’ve made_____they get hurt somehow.