The velocities of three particles of masses $20 \ g$,$30 \ g$ and $50 \ g$ are $10 \hat{i}$,$10 \hat{j}$ and $10 \hat{k}$,respectively. The velocity of the centre of mass of the three particles is

• A
  $2 \hat{i} + 3 \hat{j} + 5 \hat{k}$
• B
  $10(\hat{i} + \hat{j} + \hat{k})$
• C
  $20 \hat{i} + 30 \hat{j} + 50 \hat{k}$
• D
  $2 \hat{i} + 30 \hat{j} + 50 \hat{k}$