The open-chain aldohexose shown is drawn with wedges (it is D-glucose). Number the carbonyl carbon as C1. How many of its four stereocentres have the $(R)$ configuration? Give an integer.

At each stereocentre $\mathrm{OH}$ is first and $\mathrm{H}$ is last; the carbon group nearer the $\mathrm{CHO}$ end outranks the one nearer $\mathrm{CH}{}_2\mathrm{OH}$, because exploring towards the aldehyde end reaches a carbon with more oxygen first (at C2, $\mathrm{CHO}$ (O,O,H) beats C3 (O,C,H); at C5, C4 (O,C,H) beats $\mathrm{CH}{}_2\mathrm{OH}$ (O,H,H); at C3 and C4 the tie is broken one sphere further out in the same direction). Applying the sequence rule to the drawing gives C2 $(R)$, C3 $(S)$, C4 $(R)$ and C5 $(R)$. D-glucose is $(2R,3S,4R,5R)$, so 3 stereocentres are $(R)$.