Arabia's rainfall / average only 2-5 inches a year / two- year droughts / not uncommon.

• A. 
  Arabia's rainfall averages only 2-5 inches a year, and two-year droughts are not uncommon.
• B. 
  As Arabia's rainfall averages only 2-5 inches a year, and two year droughts are not uncommon.
• C. 
  Arabia's rainfall averages only 2-5 inches a year, which makes two year droughts are not uncommon.
• D. 
  Arabia's rainfall averages only 2-5 inches a year, and however two year droughts are not uncommon.