Read through the passage and answer the questions that follow choosing the correct alternatives.

Rain supplies most of the water that plants need, but unfortunately it does not always fall at the right time of the year or in sufficient quantities. Plants may stay alive if rain is scarce, but they will not yield such a good harvest of food. Thus, for centuries man has devised methods of supplementing natural supplies of water to his crops by means of irrigation. These methods vary according to the climate, the crops and the available water resources. In tropical and mediterranean countries, water is naturally brought to the crops by a network of surface ditches; this is known as surface irrigation. In surface irrigation water spreads over the surface and soaks into the ground.