The Food and Drug Administration is targeting salt in processed foods as a health hazard, and some experts believe the agency will require food manufacturers to limit the amount of salt in processed foods. However, the reduction will occur in phases, with salt content being reduced in steps so that consumers will be weaned off of salt gradually. Carefully controlled research studies are underway in which the salt levels are varied and consumers' reactions to the taste are measured. This research is an example of:________
a) causal research
b) conditional research
c) exploratory research
d) objective research
e) descriptive research