Volunteers for a Human Performance Study Were Randomly Divided into Two

Multiple Choice

Volunteers for a human performance study were randomly divided into two groups. The first group had their flexibility measured in the morning after a short meditation session while the second group had their flexibility measured in the afternoon with no previous meditation session. The flexibility scores of the two groups were compared.
To improve the design of this experiment, one part of it should be done in a blind way. That is, we should:

A) not tell volunteers in advance what treatment they may be exposed to.
B) not allow the volunteers to find out their flexibility scores.
C) tell all of the volunteers who meditate they will get a gym membership if they work very hard on the flexibility test.
D) be sure that the person measuring the volunteers' flexibility doesn't know which ones meditated and which ones didn't.