Which landform is most common in eastern europe and western russia?

plains

Explanation:

The largest plains of Africa occupy a vast portion of the Sahara and extend southward into the Congo and Kalahari basins. Most of the interior of Australia, from the mountains along the eastern margin to the plateaus in the far west, is one vast desert plain.

this null effect is caused by a weak manipulation and ineffective internal validity. in a scientific study, as the one dr. morimoto is trying to perfom, internal validity refers to the confidence placed in the cause-effect relationship and to how well an experiment is done and able to eliminate confounding variables.        

what are the answer choices?