Is it true that many civilizations took place in the Sahara and Sahel region of Africa in the past? if yes Mention Two civilizations which left a great impact and their achievements.

Which element about russia in 1990s in not accurate

Match the letters with the correct terms

Some gamma ray bursts are hypothesized to come from mergers of neutron stars or black holes. if this hypothesis is correct, what else should we in principle be able to detect from such mergers? a. gravitiational waves b. x-ray bursts c. significant disruption of gas thousands of light-years away in the surrounding galaxy d. an "afterglow" of bright visible light, lasting for weeks