How did the allied troops prepare for gas attacks in ww1?

When the shelling died down, the Allied defenders waited for the first wave of German attack troops but instead were thrown into a panic when chlorine gas wafted across noman’s land and down into their trenches. .The Germans targeted four miles of the front with the wind-blown poison gas and decimated two divisions of French and Algerian colonial troops. by May they had retreated to the town of Ypres

Explanation:

b is the answer i am assuming