Why is it important to test one substance at a time when doing an experiment?

Which quote from the passage best illustrates della’s optimism? “…and at length she was able to look up with dim eyes and a smile and say: "my hair grows so fast, jim! ” “…her heart had simply craved and yearned over them without the least hope of possession.” “and now, they were hers, but the tresses that should have adorned the coveted adornments were gone.” “beautiful combs, pure tortoise shell, with jewelled rims--just the shade to wear in the beautiful vanished hair.”d, ardent, most likely means? gloomy calm passionate innocent