Strontium- 90 can enter human body parts in place of

Strontium enters the blood, it flows to other parts of the body. It enters and leaves cells easily. In the body, strontium acts very much like calcium. A large portion of the strontium will build up in bones. In adults, strontium mostly attaches to the surfaces of bones. In children, strontium may create the hard bone mineral itself, thus being stored in the bones for many years. Eventually, strontium will dissolve from the bones and return to the blood to be used again to grow bone or to be expelled through urine, waste matter or sweat. The harmful effects of strontium-90 are caused by the high energy effects of radiation. 

So the correct answer is 'Calcium'.