Agroup of students designed an experiment to determine the effect of compost on the germination and growth of plants. the students set up experimental plots by mixing different ratios of soil and compost. they planted 20 pea and 20 melon seeds in each plot and watered each plot regularly. the students recorded the number of seeds that germinated, and as the plants grew, they recorded plant length, number and size of leaves, and general health observations. the following observations were recorded. in all treatments, more pea seeds germinated than melon seeds. there was a week of rainy weather at three weeks. melon plants developed a fungal growth at four weeks. melon plants grew longer than pea plants, but many melon leaves showed signs of yellowing as the weeks passed. the students concluded that pea plants grew better in compost than did melon plants. which of the following best addresses the validity of the conclusion made by the students?
(a) the conclusion is valid because the independent and dependent variables in experiment were controlled.
(b) the conclusion is valid because the experimental design included in a large sample size.
(c) the conclusion is invalid because other variables in the experiment (both biotic and abiotic) affected the results.
. (d) the conclusion is invalid because the units of measurments were not given in experimental design.