例句與造句

1. Three theories present themselves, all rooted in World War I : 1 ) an observation balloon, to get a final look at enemy trenches; 2 ) a barrage balloon, sent aloft to entangle aircraft in its dangling wires; but most likely 3 ) a signal balloon, more reliable than couriers or telephonic messages, launched to order an attack over a wide area.
2. It's difficult to find signal balloon in a sentence. 用signal balloon造句挺難的