Which of the following two changes occur in humans when they move to high altitudes (above $3,500 \ m$)?
$(1)$ Increase in red blood cell size
$(2)$ Increase in red blood cell production
$(3)$ Increase in breathing rate
$(4)$ Increase in thrombocyte count

• A
  $(1)$ and $(2)$
• B
  $(2)$ and $(3)$
• C
  $(3)$ and $(4)$
• D
  $(1)$ and $(4)$