How are biogas plants useful?

Inspired by the example of that great cornellian, vladimir nabokov, some of your friends have become amateur lepidopterists (they study butterflies). often when they return from a trip with specimens of butterflies, it is very difficult for them to tell how many distinct species they've caught— to the fact that many species look very similar to one another. solution

Who gave the famous "i have a dream" speech at the march on washington?

Why did the settlement of east texas stop in the late 1710s