What was the primary reason for these masks during world war 1

Gas masks used in World War One were made as a result of poison gas attacks that took the Allies in the trenches on the Western Front by surprise. Early gas masks were crude as would be expected as no-one had thought that poison gas would ever be used in warfare as the mere thought seemed too shocking

increases in demand are shown by a shift to the right in the demand curve. this could be caused by a number of factors, including a rise in income, a rise in the price of a substitute or a fall in the price of a complement.

which the answer is:

b