The number of chromosomes in Klinefelter's syndrome is

Read the following statements and choose the correct set of statements :
$(a)$ In sickle cell anaemia the mutant haemoglobin molecule undergoes polymerisation under high oxygen tension
$(b)$ Thalassemia is a qualitative problem
$(c)$ Turner's syndrome,Klinefelter's syndrome are common examples of chromosomal disorders.
$(d)$ Haemophilia is a sex-linked recessive disease

Sometimes chromosome number increases or decreases due to:

Which of the following disorders does not result in a sterile individual?

The chromosomal condition $2n-1$ is an example of what?

What is the cause of $Down's$ syndrome?