Directions:In this section of the test,there are ten paragraphs.Each of the paragraphs is followed by an incomplete phrase or sentence which summarizes the main idea of the paragraph.Spell out the missing letters of the word on your Answer Sheet.
Paragraph Five
We love junk food because manufacturers design it to activate brain's reward system.They create perfect combination of salt,sugar,spices which excite our taste buds.Thus activates our brain's reward system making us feel good.When we eat regular food,because it does not activate the reward system,it feels less appealing.
40.The r______ why we love junk food.
【正确答案】：reason
