Students in physics class were conducting an experiment to determine the difference in "k". They wanted to determine if displacement not only varied with mass but also with the type of spring used. The class tested two springs by hanging incremental masses from each spring and then measuring how far the spring stretched. After collecting their data, the students concluded that shorter, more tightly wrapped springs would display a smaller displacement than longer, more loosely wrapped springs. How could the students improve the validity of the data in this experiment, to better support their conclusion?

A. Use two springs made from different materials.
B. Repeat the trials several times and take an average.
C. Test a greater variety of springs; increase sample size.
D. Increase the number of mass increments they used and measured.