Which river valley civilization was conquered over and over again because it lacked natural barriers?

Eastern europe and parts of northern europe have which of the following climates?

One of the brightest comets of the 20th century was comet hyakutake, which passed close to the sun in early 1996. the orbital period of this comet is estimated to be about 30,000 years. find the semi-major axis of this comet’s orbit. compare it to the average sun-pluto distance and to the distance to alpha centauri, the nearest star to the sun, which is 4.3 light-years distant.

[fw.02]diana is studying four surface features of florida. she classifies the surface features and places them in different columns of a chart as shown below. surface features column 1 column 2 hot springs sinkholes caverns geysers which of these titles best represents column 1 of diana's chart? formed by freezing groundwater o formed by heating groundwater o formed by evaporation of groundwater o formed by chemical action of groundwater