The opening and closing of this structure on a plant leaf allows for water and gas exchange to occur. a) xylem b) phloem c) stomata d) guard cells

C) stomata

Each stoma is surrounded by two guard cells, which can open/close depending on environment. When there is moisture, the guard cells swell with water, and force the stoma to open and allow gas exchange to occur.

the correct answer is 4) heat and pressure

metamorphic rocks arise from transformation of existing rocks types, in a process called metamorphism. during metamorphism the original rock(prolith) change into metamorphic rock by heating (150-200c) and pressure of more than 1000 bar. the original rock can be sedimentary, igneous, or existing metamorphic rock.