Read following passage and answer item that follow.
An earthquake comes like a thief in the night, without warning. It was necessary, therefore, to invent instruments that neither slumbered nor slept. Some devices were quite simple. One, for instance, consisted of rods of various lengths and thicknesses which would stand up on end like ninepins. When a shock came it shook the rigid table upon which these stood. If it were gentle, only the more unstable rods fell. If it were severe, they all fell. Thus the rods by falling and by the direction in which they fell, recorded for the slumbering scientist.
But, instruments far more delicate than that were needed if any really serious advance was to be made. The ideal to be aimed at was to devise an instrument that could record with a pen on paper the movements, of the ground or of the table, as the quake passed by. But when table, penholder and paper are all moving how is it possible to write legibly ? The key to a solution of that problem lay in an everyday observation. Why does a person standing in a bus or train tend to fall when a sudden start is made ? It is because his feet move on, but his head stays still.
The everyday observation referred to in the passage relates to :

Options

A. A) a moving bus or train.
B. B) the sudden start of a bus.
C. C) the tendency of a standing person to fall when a bus or train
moves suddenly
D. D) people standing in a bus or train.