The correct answer is C) Highly explosive gases will otherwise accumulate. During the charging of storage batteries, hydrogen gas is produced as a byproduct. This hydrogen gas is highly flammable and can accumulate to explosive levels in poorly ventilated battery rooms. Proper ventilation is required to disperse the hydrogen gas and prevent the buildup of this explosive hazard. The other answer choices are incorrect - option A is not the primary reason for ventilation, option B refers to excessive gassing rather than explosive gas buildup, and option D is incorrect as the gases released are not highly poisonous, but rather flammable.