Manuel is performing a scientific investigation to determine whether fertilizer a or fertilizer b is more effective at increasing the growth rate of tomato plants. manuel measures and separates 30 tomato plants into two equal groups that are planted in identical environments. he gives one group fertilizer a, and gives the other group the same amount of fertilizer b. during the first week of the investigation, he gives the tomato plants in both groups the same amount of water. during the second week, however, he forgets to water the tomato plants being given fertilizer b, but still provides water for the tomato plants being given fertilizer a. two weeks after planting, manuel measures the tomato plants and discovers that the tomato plants given fertilizer a have grown much more rapidly than the tomato plants being given fertilizer b. manuel's results

a. are not valid because one of the variables was not controlled during week 2.

b. are valid because he controlled all of the variables in week 1.

c. are valid because he designed his experiment correctly.

d. are not valid because he should not have grown the plants in identical environments.