Arthritis is the disease of

Which of the following is a disorder of the muscular/skeletal system?

Given below are two statements: one is labelled as Assertion $(A)$ and the other is labelled as Reason $(R)$.
Assertion $(A)$:
Osteoporosis is characterised by decreased bone mass and increased chances of fractures.
Reason $(R)$:
Common cause of osteoporosis is increased levels of estrogen.
In the light of the above statements,choose the most appropriate answer from the options given below:

Match List-$I$ with List-$II$ and select the correct option.

List-$I$	List-$II$
$a$. Tetany	$i$. Autoimmune disorder
$b$. Osteoporosis	$ii$. Progressive degeneration of skeletal muscle
$c$. Muscular dystrophy	$iii$. Inflammation of joints
$d$. Arthritis	$iv$. Rapid spasms in muscle
$e$. Myasthenia gravis	$v$. Decreased bone mass

Osteoporosis,an age-related disease of the skeletal system,may occur due to:

Old people are more liable to fracture of their bones because