The paragraph is followed by an incomplete phrase or sentence which summarizes the main idea of the paragraph. Choose the missing words of the phrase or sentence.

The principle was discovered in 1809 by a Frenchman called Nicolas Appert. He corked food lightly in wide-necked glass bottles and immersed them in a bath of hot water to drive out the air, then he hammered the corks down to seal the jars hermetically. Appert's discovery was rewarded by the French government because better preserved food supplies were needed for Napoleon's troops on distant campaigns.
The （） and early methods of preserving food.