In this task, find the most appropriate number to replace the blank (indicated with _ ) and express it in words.

Q: Optical isomers differ in the arrangement of _ groups around a chiral carbon.

A: four
****
Q: Roaches have two forewings and _ hind wings.

A: two
****
Q: Bubonic plague begins in China and India, killing an estimated _ million over the next decade.

A:
three
****