Rylan is testing 2 different types of nail polish on her fingernails to determine which one lasts longer. she purchases brand a and brand b from the drug store. she then paints her nails with brand a and her sister's nails with brand b. she compares the nail polish at the end of the week and determines that brand b lasts longer because there are fewer chips than brand a. why is her conclusion not valid?

she painted her sister's nails with brand a, not her own nails.
she painted her sister's nails with brand b, not her own nails.
she did not say how much each polish chipped by the end of the week.
she purchased 2 different types of nail polish.