Answer:
One must raise food (grain, corn) for the cow to eat before it is fully grown.
They used more than one bomb but to answer your question, They tested in present day Manhattan before releasing on Hiroshima and Nagsaki.
Ancient civilizations grew around river valleys because the land around river valleys was lush and fertile, allowing effective systems for growing crops. Also helping with the crop growth, the rivers would naturally flood just enough to keep the soil properly moist. The rivers also served as a sure-fire and efficient way to trade with other civilizations. Because of these factors of land and trade, many ancient civilizations sprung up around river valleys.