Passage 3
Questions 11 to 15 are based on the following passage:
The vitamins necessary for a healthy body are normally supplied by a good mixed diet (饮食) , including a variety of fruits and green vegetables. (79) It is only when people try to live on a very restricted diet that it is necessary to make special provision to supply the missing vitamins.
An example of the dangers of a restricted diet may be seen in the disease known as“beri-beri”. (80) It used to distress large numbers of Eastern peoples who lived mainly on rice. In the early years of this century, a scientist named Eijkman was trying to discover the cause of “beri-beri”. At first he thought it was caused by a germ. He was working in a Japanese hospital, where the patients were fed on polished rice which had the outer husk (外壳) removed from the grain. It was thought this would be easier for weak and sick people to digest.
Eijkman thought his germ theory was confmned when he noticed the chickens in the hospital yard, which were fed on leftovers
(剩饭) from the patients' plates, were also showing signs of the disease. He then tried to isolate the germ, but his experiments were interrupted by a hospital official, who declared that the polished rice, even though left over by the patients, was too good for chickens. It should be recooked for the patients, and the chickens should be fed on cheap rice with the outer layer still on the grain.
Eijkman noticed that the chickens began to recover on the new diet. He began to consider the possibility that eating unpolished rice somehow prevented or cured "beri-beri" -- even that a lack,of some element in the husk might be the cause of the disease. Indeed thiswas the case. The element needed to prevent “beri-beri” was shortly afterwards isolated from Ace husks and is now known as vitamin B. Nowadays, this terrible disease is much less common thanks to our knowledge of vitamins.
11. A good mixed diet______.

A. normally contains enough vitamins
B. still needs special provision of vitamins
C. is suitable for losing weight
D. is composed of fruits and vegetables