Please send help also 20points

An echo occurs when a hits something hard?

im thinking its a soundwave but im not sure

ill mark brainliest if its the right answer

sound wave

Explanation: Echo is a sound heard after a sound wave hit a hard surface. The sound is what is heard back as echo. An echo doesn't occur when the sound wave have been absorbed before hitting an hard surface so in a large crowd you might not hear an echo because the sound wave have been absorbed. The echo is heard faster when you are standing close to a hard surface than when you are far away a hard surface.

coconut why: has water in it : )

the truman doctrine was an american foreign policy whose stated purpose was to counter soviet geopolitical expansion during the cold war. it was announced to congress by president harry s. truman on march 12, 1947, and further developed on july 12, 1948, when he pledged to contain threats to greece and turkey.