Correct answer: b
There are many elements in human body, and the main elements are calcium, magnesium, potassium, sodium, phosphorus, sulfur and chlorine. Trace elements include iron, zinc, iodine, copper, molybdenum, chromium and cobalt, which are all essential components for human body, and the lack of one of them will cause diseases. "Calcium, fluorine and zinc" should be understood as elements.