With reference to the ground level, the air – fuel ratio at this altitude will be

Question

An air breathing aircraft is flying at an altitude where the air density is half the value at ground level. With reference to the ground level, the air – fuel ratio at this altitude will be                         

      (a) $\sqrt[3]{2}$                                                                  (b) $\sqrt{\frac{1}{2}}$ 

      (c) 2                                                                        (d) 4