A randomized controlled trial investigated whether an exercise program increased activity levels in women. Intervention consisted of brief physical activity with regular support that was led by nurses. Control participants received usual care. The order in which participants were allocated to treatment was contained in sequentially numbered opaque envelopes. A researcher not involved in recruitment generated the sequence by computer. Following written informed consent and baseline measurements, treatment allocation was revealed by a nurse not involved in future assessments. Participants were assessed at follow-up by nurses unaware of group allocation; assessors and participants were asked not to discuss group allocation.
Which of the following, if any, are true?
a) Allocation concealment was achieved
b) The trial was single blind
c) The trial was double blind
d) Selection bias was minimized