Which sentence is a good opening topic sentence for the following text?
vitamin d the body absorb calcium and phosphorous. it assists the body by increasing the amount of calcium absorbed from the small intestine. this in turn forms and maintains bones. vitamin d plays a crucial role in immunity and controlling cell growth. it protects the body like a soldier and the body fight infections. it is important that children get adequate amounts of vitamin d to develop healthy teeth and bones. the main source of vitamin d is milk. vitamin d is also found in herring, salmon, and sardines. cod liver oil is another source of vitamin d. our skin also produces vitamin d from sunlight. however, for the skin to absorb vitamin d from sunlight, it needs to be exposed to sunlight. about 15 minutes of exposure twice a week is enough for the skin to produce vitamin d. the skin can be exposed to the sun while taking a walk or playing outside.