Which of these is an agent of solution? A. Rock pieces
B. Acid rain
C. High pressures
D. Plant roots

answer; ///(b) it is composed of four protein chains and four heme groups;

these muscles maintain the blood pressure in the vessels. the smooth muscles are innervated by nerves of the autonomic nervous system. when the muscles contract, they cause vasoconstriction of the vessels and increase blood pressure. when the muscles relax, they enable vasodilation of the   blood vessels hence reduce blood pressure.  

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