Which of the Following Represents a Blind Study

Which of the following represents a blind study?

A) The test group knows whether they are receiving a placebo or the actual treatment, and so do the researchers.
B) The test group does not know whether they are receiving a placebo or the actual treatment, but the researchers do.
C) The test group knows whether they are receiving a placebo or the actual treatment, but the researchers do not know until all the data are collected.
D) Neither the test group nor the researchers know whether the test group is receiving a placebo or the actual treatment until all the data are collected.