It is difficult to achieve global sustainability because A. it is an unrealistic goal.
B. governments do not always agree on how to solve environmental problems.
C. international agreements are not binding.
D. globalization is increasing.

since there are no options posted i'll give you a few examples.

a dolphin's flipper, bird's wing, cat's leg.

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.