A open ended mercury manometer is used to measure the pressure exerted by a trapped gas as shown in the figure. Initially manometer shows no difference in mercury level in both column as shown in diagram.

After sparking the container, 'A' dissociates according to following reaction

$\mathrm{A}\left(\mathrm{g}\right)⟶\mathrm{B}\left(\mathrm{g}\right)+3\mathrm{C}\left(\mathrm{g}\right)$

If pressure of Gas "A" decreases to 0.9 atm, then (Assume temperature to be constant and is 300 K) the wrong statement is