When Does an Individual Typically Experience Their First Period of REM

When does an individual typically experience their first period of REM?

A) Immediately after falling asleep, and before entering non-REM
B) About 4 hours into sleep, after experiencing 2 or 3 cycles of stages 1 through 3
C) About 2 hours after falling asleep, and have already gone in and out of stages 1, 2, and 3
D) Approximately 1 hour after falling asleep, and experiencing only stage 1 sleep previous to REM