Select the option that can be used as a one-word substitute for the given group of words. A period of ten years

Correct — B. A decade is a period of ten years. The word comes from Greek dekas, a group of ten, and shares its sense with the Latin decem behind decimal and December.

All four options name time spans, so the item is a straight test of which number root you can read: two, five, ten or a hundred. Ten belongs to option (b).

Why the others are wrong

• (a)
  Biennium — A biennium is a period of two years, from Latin bi-, twice, and annus, year. The same root gives biennial, an event that happens every two years.
• (c)
  lustrum — A lustrum is a period of five years — in ancient Rome the interval between the purification ceremonies held after each census. Half a decade, not a decade.
• (d)
  Century — A century is a period of a hundred years, from Latin centum, a hundred. It is ten decades, so it overshoots the clue by a factor of ten.

Time-span substitutes are pure vocabulary, but they are learnable as a ladder because almost every rung is built on a number word.

Decade is the ten-year rung. Biennium sits at two years below it and century at a hundred years above it, with millennium at a thousand above that.

Learn the root and the family becomes derivable — bi is two, decem is ten, centum is a hundred, mille is a thousand. Lustrum is the exception, named after a Roman ceremony rather than a number.

Key facts

• Decade is ten years, century a hundred years and millennium a thousand years.
• Biennium is two years, and a biennial plant completes its life cycle over two growing seasons.
• Lustrum is five years, named for the purification ceremony held in Rome after each census.
• Biennial means every two years, while biannual means twice in one year.

Common traps

• Mixing up biennial, every two years, with biannual, twice a year.
• Treating lustrum as the unfamiliar word that must therefore be the answer.

No directly related past PYQ was found.