Questions

Astronomy is the study of planets and heavenly bodies. Historically, astronomy has focused on observations of heavenly bodies. It is a close cousin to astrophysics. Succinctly put, astrophysics involves the study of the physics of astronomy and concentrates on the behavior, properties and motion of objects out there.
 

Since, the starting statement carries a subject; R will be the first statement. The next statement describes why are they hunted, so Q follows P, as it states that what is being done with the horns. S would be the concluding statement and thus form a coherent paragraph.

The correct formation would be, ‘The Rhinoceros is hunted by poachers for its horn which is sold at high prices.’

Tooth Enamel is the hardest and most highly mineralized substance in the human body. It is a tissue and not a bone. This tissue has high mineral content which makes it hardest substance.

Femur and Temporal bone of the skull are the hardest bones of the human body.