The user wants to know the answers to questions 16, 17, and 18 regarding vitamins and their sources. Questions are multiple choice.

Here are the answers to the questions:

Question 16:
The correct option is C.
Vitamin B (referring to the B-complex vitamins like B1, B2, B3, B5, B6, B7, B9, and B12) are well-known for their crucial role as coenzymes in various metabolic processes, particularly in energy production. While Vitamin C also acts as a coenzyme in some reactions, the B vitamins are the most extensive group primarily recognized for their coenzyme functions. Vitamin A does not function as a coenzyme.

Question 17:
The correct option is A.
Sweet potato is an excellent source of beta-carotene, which is a precursor that the body converts into Vitamin A. Other good sources of Vitamin A include liver, carrots, and leafy green vegetables. Dairy products also contain Vitamin A, but sweet potato is often highlighted for its high content of provitamin A.

Question 18:
The correct option is D.
Vitamins are classified as either fat-soluble (A, D, E, K) or water-soluble. Vitamin B1, Vitamin C, and Vitamin B2 are all examples of water-soluble vitamins. Water-soluble vitamins are not stored in the body to a significant extent and are excreted in urine.