Low level of $Ca^{2+}$ ions in muscles results in

Deficiency of which vitamin causes bow-leggedness in children?

Which of the following is incorrect regarding osteoporosis?

Osteoporosis,an age-related disorder of the skeletal system,is characterized by:

Match the following columns and select the correct option:

Column $I$	Column $II$
$(a)$ Gout	$(i)$ Decreased levels of estrogen
$(b)$ Osteoporosis	$(ii)$ Low $Ca^{2+}$ ions in the blood
$(c)$ Tetany	$(iii)$ Accumulation of uric acid crystals
$(d)$ Muscular dystrophy	$(iv)$ Autoimmune disorder
	$(v)$ Genetic disorder

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: