$A$ polymer sample contains $10$ molecules each with molecular mass $5,000$ and $5$ molecules each with molecular mass $50,000$. The number average molecular mass of the polymer sample is

• A
  $2 \times 10^4$
• B
  $3 \times 10^4$
• C
  $2 \times 10^5$
• D
  $3 \times 10^5$