Which two of the following changes $(i-iv)$ usually tend to occur in plain dwellers when they move to high altitudes ($3,500 \ m$ or more)?
$(i)$ Increase in red blood cell size
$(ii)$ Increase in red blood cell production
$(iii)$ Increased breathing rate
$(iv)$ Increase in thrombocyte count
Changes occurring are

• A
  $(ii)$ and $(iii)$
• B
  $(iii)$ and $(iv)$
• C
  $(i)$ and $(iv)$
• D
  $(i)$ and $(ii)$