utilids are marine mollusks that begin growing their shells before nerging from their eggs and continue to add shell segments throughout eir lifetimes. The walls between their shells' chambers are called septa, nd the concentration of the isotope oxygen-18 they contain exactly reflects the isotope's concentration in the water at the depth at which the septa formed. Paleontologist AmaneTajika and colleagues examined each of the septa in two nautilid shells; finding that sample F13 had a higher concentration of the isotope than sample MO3 did the researchers concluded that F13 must have formed at a significantly lower temperature than M03 did. 13 W Mark for Review Which finding, if true, would most directly support the researchers' conclusion? A As water temperature increases nautilids are able to concentrate more oxygen-18 into their shells. B As the concentration of oxygen -18 in water decreases, water temperature decreases and nautilidid shell growth slows. C As water depth increases both the concentration of oxygen-18 and the water temperature decrease. D As water depth increases the concentration of oxygen-18 increases and water temperature decreases.

To support the researchers' conclusion that sample F13 formed at a significantly lower temperature than sample M03, we need to find a statement that links higher concentrations of oxygen-18 in nautilid shells with lower water temperatures.<br /><br />Let's analyze each option:<br /><br />A. As water temperature increases, nautilids are able to concentrate more oxygen-18 into their shells.<br />- This suggests that higher temperatures lead to higher concentrations of oxygen-18, which contradicts the researchers' conclusion. Therefore, this does not support the conclusion.<br /><br />B. As the concentration of oxygen-18 in water decreases, water temperature decreases and nautilid shell growth slows.<br />- This implies that lower concentrations of oxygen-18 are associated with lower temperatures, which is opposite to what the researchers found (higher oxygen-18 concentration in F13). Thus, this does not support the conclusion.<br /><br />C. As water depth increases, both the concentration of oxygen-18 and the water temperature decrease.<br />- This suggests that deeper water has lower temperatures and lower concentrations of oxygen-18. Since F13 had a higher concentration of oxygen-18, this would imply it was formed in shallower, warmer water, which contradicts the researchers' conclusion. Therefore, this does not support the conclusion.<br /><br />D. As water depth increases, the concentration of oxygen-18 increases and water temperature decreases.<br />- This indicates that deeper water has higher concentrations of oxygen-18 and lower temperatures. If F13 had a higher concentration of oxygen-18, it would suggest it formed in deeper, colder water, supporting the researchers' conclusion.<br /><br />Therefore, the finding that most directly supports the researchers' conclusion is:<br /><br />D. As water depth increases, the concentration of oxygen-18 increases and water temperature decreases.