The doppler effect refers to the frequency as perceived by a. the source of the waves b. an observer of the waves who is moving relative to the source c. an observer of the waves who is stationary relative to the source d. someone measuring halfway between the source and the observer, regardless of relative motion

In which situation has the independent variable for the experiment been changed? check all that apply. changed from counting the number of duckweed at day 0 and day 14 to counting them every day changed from testing duckweed growth in beakers to testing duckweed growth in large outdoor tanks changed from testing the effect of ph on duckweed growth to testing the effect of light levels on duckweed growth changed from testing the effect of acid ph on duckweed growth to testing the effect of basic ph on duckweed growth