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which statement describes the condition of the heart at the point indicated in the electrocardiogram?

a. the atria are contracting and pushing blood into the bottom ventricles of the heart.

b. the valves of the aorta and pulmonary artery close, producing the first heartbeat.

i'm thinking (a) to keep sand in place; ////////////

explanation: a sand fence or sandbreak, like a snow fence, is used to force windblown, drifting sand to accumulate in a desired place. sand fences are employed to control erosion, sand dune stabilization, keep sand off roadways, and to recruit new material in desert areas.