Question

In World War I, both submarines and zeppelins allowed combatants to  bomb enemy trenches  deliver large numbers of troops  reduce marine traffic  spy on the enemy

Answer 1

I think to spy

Step-by-step explanation:

Answer 2

Spy on the enemy since submarines couldn't possible bomb enemy trenches or deliver a large number of troops. Although a zeppelin could bomb a few ships, they could also be used to spy on the enemy just like a submarine. I'm fairly certain they allowed combatants to spy on the enemy although I could be wrong and they were used to reduce marine traffic.