Education about sex is very important.

He is majoring in chemistry at university.

What is this material made of?

He is an expert in chemistry.

I can't drive the car because there is a shortage of gasoline.

His body showed a rejection reaction to the transplanted organ.

His single word triggered a chain reaction, and everything changed.

She overreacted to his words.

Nuclear reactions are used for mass production of energy.

I was surprised by his overreaction.