Which combination will produce a precipitate? agno3 (aq) and ca(c2h3o2)2 (aq) naoh (aq) and hcl (aq) nacl (aq) and hc2h3o2 (aq) nh4oh (aq) and hcl (aq) fe(no3)2 (aq) and koh (aq)?

The combination that will produce a precipitate is Fe(NO₃)₂(aq) and KOH(aq). The balanced chemical equation shows the production of iron(II) oxide, Fe(OH)₂(s) and potassium nitrate, KNO₃.

Fe(NO₃)₂(aq) + 2KOH(aq) → Fe(OH)₂(s) + 2KNO₃(aq)

Fe(OH)₂ is insoluble in water. It is naturally a white solid but in the presence of even a small amount of oxygen, it turns into a green color. That is why, it is also referred to as "green rust". The other product KNO₃ is an ionic salt that forms an aqueous solution in water. 

Other combinations among the choices do not produce a solid. This is based on the solubility rules wherein it is stated that:

1. All compounds of C₂H₃O₂, Na⁺, NH₄⁺ along with other alkali metals are soluble in water.
2. All compounds with Cl are soluble except for Pb, Hg, and Ag.

Organisms are more likely to be more preserved in mud than sand because mud is moist and wet, while sand is very dry.

They carry out the exchange of gases as we breathe ( oxygen and carbon dioxide).