When do organs and organ systems begin to develop in human? A. In the sperm cell before fertilization.
B. Before fertilization and after birth.
C. In the egg cell after fertilization.
D. After fertilization and before birth​

D. After fertilization and before birth​

Explanation:

At Six Weeks: The embryo grows to about 1/3- inch (less than one centimetre). The spinal cord, brain, ears and eyes have started to develop. The internal organs (lungs, digestive system, kidneys) begin to appear.

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before, trains worked using the steam engine. thus, it uses heat so it is crucial to apply heat balance. the steam engine absorbs energy during the stage of boiling water. so, this is the endothermic reaction. when the steam is used to work the piston, it has to release this energy through running the train. the excess energy is released as an exhaust to the chimney. this is the exothermic process because it releases energy to the surroundings.