Write the correct form of the verb in the blank. water (boil) at 100 c. an astronomer (observe) the universe through optical and radio telescopes. red blood cells (circulate) in the circulatory system. the corrosion of silver (produce) silver sulphide. a rheostat (vary) the current in a circuit. the stomata in a leaf (open) and (close) to allow the exchange of oxygen for carbon dioxide. children usually (resemble) their parents. a well-equipped laboratory (supply) water, air and steam. many international students (major in) in technological fields. the nervous system (be) a mechanism by which an animal (perceive) and (respond to) its environment.