The heart valves act like doors.

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All living organisms are composed of a. only one cell. b. at least 100 cells. c. at least three cells. d. one or more cells.

This is a situation in which genes are attached to an organism's sex chromosomes; the sex of an organism influences the expression of a gene.

Kudzu vines grow by climbing and wrapping around trees. trees covered by kudzu can die because they are starved of sunlight. what type of relationship exists between the trees and the kudzu growing on them?