試題詳解

23.The body gets most of its vitamin D not from diet but from skin exposed to the ultraviolet B (UVB) radiation in sunlight.
(A) Unprotected skin on the arms and legs need about 15 minutes of sun exposure a day in spring, summer, and fall to make enough of the vitamin.
(B) Vitamin D deficiency causes rickets, a disease characterized by a failure of bone tissue to properly mineralize, resulting in soft bones and skeletal deformities.
(C) However, the production of vitamin D is effectively blocked if you follow current advice to prevent skin cancer by using ample amounts of sunscreen.
(D) Experts now advise people not to use too much sunscreen because it may completely block UVB radiation and prevent synthesis of vitamin D.