选择题：Questions 6 to 10 are based on the following passa

Questions 6 to 10 are based on the following passage or dialog. In the early days of sea travel, seamen on long voyages lived exclusively on salted meat and biscuits. Many of them died of scurvy (坏血病), a blood disease which causes swollen gums, livid (铅色的) white spots on the flesh and general exhaustion. On one occasion, in 1535, an English ship arrived in Newfoundland with its crew seriously ill. The men.s lives were saved by Iroquois Indians (易洛魁印第安人) who gave them vegetable leaves to eat. Gradually it came to be realized that scurvy was caused by something lacking in the sailors. diet. Captain Cook, on his long voyages of discovery to Australia and New Zealand, established the fact that scurvy could be warded off by the giving fresh fruit to the sailors. Nowadays it is understood that a diet which contains nothing harmful may yet result in serious diseases
if certain important elements are missing. These elements are called "vitamins". Quite a number of such substances are known and they are given letters to identify them, A, B, C, D, and so on. Different diseases are associated with a lack of particular vitamins. Even a slight lack of Vitamin C, for example, the vitamin most plentiful in fresh fruit and vegetables, is thought to increase our chances of catching colds. The vitamins necessary for a healthy body are normally supplied by a good mixed diet, including a variety