Read the passage from "Zero-Gravity Nutrition.” It has long been a challenge to decide on astronauts’ food. Astronauts don’t have the choice of foods that are on Earth. In early space travel, the solution was to put food in tubes. Tubed food probably sounds strange to you! Today, foods are chosen to travel and keep. For example, foods such as nuts and dried fruit can be eaten in space just as they are.

Which word in the passage is most repeated, thus hinting that it’s part of the central idea?

challenge
food
space
tubes