Effect of Calcium Supplements on Bone Density

Argument
Scientists wanted to see whether giving pre-puberty children dietary calcium supplements could significantly increase the children's bone density. They selected 71 pairs of identical twins and gave one twin of each pair a daily supplement and gave the other twin a placebo. All twins had the same diet otherwise over three years. At the end of the three years, the scientists found that the twins who had received extra calcium had significantly greater bone density. Extra calcium caused the increased density.
For your assigned argument, answer the following questions:
A) In a single statement, what is the effect being investigated?
B) In a single statement, what is the suspected cause of the effect?
C) Does the argument use the Method of Agreement, Method of Difference, both or none (just correlation).
C) Evaluate the strength of the argument (strong/weak) and explain your answer in one statement. Remember that strength is about inductive rationality, which is not the same thing as truth.