Can someone help me please

When the mercury level in a barometer decreases that atmospheric pressure has

4al + 3o2 → 2al2o3 what does the "3" in front of o2 stand for? a) it indicates that there are 5 oxygen atoms after you add the coefficient and the subscript. b) it indicates that that there are are total of 6 oxygen atoms all bonded together as a single molecule. c) it indicates that there are 3 oxygen molecules chemically bonded to each other in the reaction. d) it indicates that there are 3 separate oxygen molecules in the reaction.

Which correctly identifies the parts of a transverse wave? a: crest b: amplitude c: wavelength d: trough a: trough b: amplitude c: crest d: wavelength a: trough b: amplitude c: wavelength d: crest a: crest b: amplitude c: trough d: wavelength