Answering Science Questions without Science

Sometimes you can answer Science questions based purely on the units in the answers.  In the following question, knowing what S is or what Table 2 says is unnecessary.

34. Lightning strikes can occur when air between the ground and the base of a storm cloud is exposed to an electric field stronger than S for air. Suppose a certain storm cloud's based is 300m above the ground the ground. Based on Table 2, approximately what minimum voltage would be present between the storm cloud's base and the ground immediately before a bolt of lightning from this storm cloud strikes the ground? 

F.  300,000 mm x 3.13 kV/mm 
G. 300.000 m x 3.13 kV/mm
H.    300 mm    
     3.13 kV/mm
J.       300 m     
    3.13 kV/mm

December 2015 (72E)

The question asks for "minimum voltage." Therefore, the unit of the answer will be have to be some sort of voltage, and the only unit of voltage is kV.. Get rid of H and J because kV is in the denominator, so the unit in those will be 1/kV. Get rid of G because it has different length units in the  numerator (meters - m) and the denominator (millimeters -mm), and they won't cancel out. The only answer in which the length units will cancel, leaving a voltage unit, is F.