Yuan Longping realised that larger fields were not the solution. Instead, farmers needed to boost yields in the fields they had. How this could be done was a     1     (challenge) question at the time. Yuan was convinced that the answer could be found in the creation of hybrid rice. One characteristic of hybrids is     2     they usually attain a higher yield than conventional crops. However,     3     it was possible to develop a hybrid of self-pollinating plants such as rice was a matter of great debate.

Your teeth may be part of your smile,     1    they have a more important job — they are the first step in the process of digestion. You have three     2    (kind) of teeth. In the very front of your mouth are eight flat, thin teeth called incisors. They are used for cutting and biting food. Next to these are pointed teeth called canines,     3     also help you tear food. The remaining teeth are molars. These teeth have flat tops for crushing and grinding the food.