you are performing a biochemical assay of b-galactosidase activity. test tube a contains 5.0 ml of luria broth (lb). test tube b contains 4.6 ml of lb and 400 ul of 4% lactose. you pipette 250 ul of e. coli bacteria into each test tube, mix them, and incubate them for 20 minutes. after this incubation period, you retrieve two microcentrifuge tubes and the provided b-gal blank. you remove 500 ul each from test tubes a and b and add it to your labeled microcentrifuge tubes, then add 10 ul of popculture and allow it to incubate before adding 400 ul of z-buffer mix. later in the assay, you will measure the optical densities of these mixtures at 420nm after adding the appropriate substances (onpg, and na2co3) and closely following the procedures. which volume would you use in your calculation of units of enzyme activity?