Answer:
B
Step-by-step explanation:
B. Identical twins to study.
Identical twins are useful in this research because they have the same genetic makeup, so any differences in bone density between them would likely be due to environmental factors, rather than genetic differences. This would allow you to isolate the effect of environmental factors on bone density, and study them separately from the effects of genetics.
A mass spectrometer would be useful for studying the chemical composition of bone samples, but would not be as directly relevant to determining the effect of environmental factors on bone density.
An X-ray machine would be useful for measuring bone density, but would not be as useful for isolating the effect of environmental factors, since it does not provide information about genetics or other possible causes of differences in bone density.
Siblings would also be useful to study, but identical twins would be more useful since they share the exact same genetic makeup, which will make it easier to isolate environmental factors' effect on bone density.