Join the sentences in two ways, with or without which/that.

a) A lot of rubbish is floating in the sea. It is a real danger to health.
b) The problem of soil erosion is discussed in this chapter. It has occurred ever since the man began destroy forests.
c) Nuclear energy comes from the energy stored within the nuclei of atoms. They are the basic building blocks of all matter.
d) The best things in life such as water and air are free. We often take it for granted.
e) The manufacture of one PC requires 28kg of liquid chemicals. All liquid chemicals have to be neutralized by a further 11 kg of sodium hydroxide.