单选题 .    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 disease of the blood which causes swollen gums, livid white spots on the flesh and general exhaustion. On one occasion, in 1535, an English ship arrived in Newfoundland with its crew desperately iii. The men's lives were saved by Iroquois Indians who gave them vegetable leaves to eat. Gradually it came to be realized that scurvy was caused by some lack in the sailors' diet and Captain Cook, on his long voyages of discovery to Australia and New Zealand, established the fact that scurvy could be warded off by the provision of fresh fruit for the sailors.
    Nowadays it is understood that a diet which contains nothing harmful may yet result in serious disease 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 deficiencies 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 significantly our susceptibility(敏感度) to colds and influenza.
    The vitamins necessary for a healthy body are normally supplied by a good mixed diet, inclu- ding a variety of fruit and green vegetables. It is only when people try to live on a very restricted diet, say during extended periods of religious fasting(斋戒), or when trying to lose weight, that it is necessary to make special provision to supply the missing vitamins.1.  Scurvy is a disease that is provoked by ______.