Answer:
Double blind.
Step-by-step explanation:
Double blind study may be defined as the experiment in which neither the experimenters nor the participants are aware about the particular treatment. This provide bias in the research.
The scientists do not know which yogurt contains the bacteria. They divide the people into two groups. Here, neither the researcher nor the participants know which yogurt contains bacteria. This is an example of the double blind research.
Thus, the answer is double blind.