A、Because they need more protein than vegetables supply. B、Because they cannot digest vegetables. C、Because they use more energy

Whether or not vegetarianism should be advocated for adults，(26)it is definitely unsatisfactory for growing children，who need more protein than they can get from vegetable sources．Vegetarianism is just not healthy for them．
However，a lacto-vegetarian diet，which includes milk and milk products such as cheese，can be satisfactory as long as enough milk and milk products are consumed．
Meat and cheese are the best sources of usable animal protein and next come milk，fish and eggs．
(27)Slow and careful cooking of meat makes it more digestible and assists in the breaking down of the protein content by the body．However，when cooking vegetables，the vitamins，and in particular the water-soluble vitamin C，should not be lost through over cooking．
With fruit，vitamin loss is negligible．because the cooking water is normally eaten along with the fruit，and acids in the fruit help to hold in the vitamin C．
Most nutrition experts today would recommend a balanced diet containing elements of all foods，largely because of our need for sufficient vitamins．Vitamins were first called“accessory food factors”since it was discovered，in 1906，that most foods contain，besides carbohydrates，fats，minerals and water，these other substances necessary for health. The most common deficiencies in Western diets today are those of vitamins．The answer is variety in food．(28)A well-balanced diet having sufficient amounts of milk，fruit，vegetables，eggs，meat and fish usually provides adequate minimum daily requirements of all the vitamins．