Why a zigzag?

Have you ever noticed that when we climb hills we create zigzags? We don’t go straight ahead but go in zigzags. It can mean walking 20 times as far, and still a zigzag is (FAST) than the shortest distance between two points.
Researchers developed a mathematical model (SHOW) that a zigzag provides the most efficient way for humans to go up or down steep slopes. They explain that zig-zagging (REQUIRE) less effort.

Russian rivers

Do you know Russian rivers? Do you know what the (LONG) river in Russia is? It is the Volga River. This river is a great source of hydroelectric power. The water from the river (USE) for irrigation. Canals link the Volga River to the Baltic and White Seas. The Volga (FREEZE) for most of its length for three months each year.