Hello there!
I think the answer would be - D.
There needs to be some type of Casebo (sugar pill), that can be used. If t he y are just testing the vaccine, then all the children can "think" they are getting better, but the vaccine in reality, would be doing nothing. This is a good reason why Casebos should always be used.
Hope this helps!
P.S. Sorry for any spelling errors!
~DL ☆☆☆