For his science project, austin wants to determine if the ph of standing rainwater changes over time. he collects a jar of rainwater and places it indoors in a spot that does not receive a lot of sunlight to limit evaporation. using a ph testing kit, he measures the ph of the water once a day for 20 days.

what is the independent, or manipulated, variable in austin’s experiment?

a. the number of days the rainwater is allowed to sit
b. the number of times the rainwater is observed
c. the change in the amount of rainwater due to evaporation
d. the change in ph of the rainwater over time

Match the following terms describing electrical events with the correct phases of the cardiac cycle. the ventricular muscle cells depolarize at the start of this phase. during this phase, na+ entry through the funny channels causes the pacemaker potential of sa nodal cells to gradually become less negative until it reaches threshold. the ventricular muscle cells repolarize right before this phase. the cytosolic concentration of calcium in the contractile cells of the ventricle is highest during this phase. a. isovolumetric relaxation b. ventricular ejection c. isovolumetric contraction d. ventricular filling

The table shows dates and appearance of index fossils. which rock layers can be dated most precisely? a) a layer containing both a fossil 4 and a fossil 3 b) a layer containing both a fossil 2 and a fossil 3 c) a layer containing both a fossil 1 and a fossil 4 d) a layer containing both a fossil 1 and a fossil 2

Which of the following is a consequence of urban heat islands? increased precipitation downwind of the city? increased precipitation upwind of the city? decreased winds within the city? increased winds within the city?