The purple sulfur and green sulfur bacteria a. both use hydrogen sulfide as a source of electrons. b. generate oxygen. c. preferentially use organic molecules as an electron source. d. both lack gas vesicles.

A. both use hydrogen sulfide as a source of electrons

Explanation:

Both purple and green sulfur bacteria exhibit anoxygenic photosynthesis and do not produce water as a by-product of the process. During the process, they use any organic or inorganic compound as a source of the electron but do not use water molecules for the same purpose.

Both of these bacteria preferentially use H2S (hydrogen sulfide) as an electron donor and oxidize it into elemental sulfur or sulfates.

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i'm thinking (a) to keep sand in place; ////////////

explanation: a sand fence or sandbreak, like a snow fence, is used to force windblown, drifting sand to accumulate in a desired place. sand fences are employed to control erosion, sand dune stabilization, keep sand off roadways, and to recruit new material in desert areas.