Select the correctly spelt word to fill in the blank. Helium is used with oxygen for asthma treatment because it ____________ very easily.

Correct — C. The word is spelt diffuses — a double f, and an s in the ending, never a z.

The double f is inherited: Latin dis- assimilates to dif- before the f of fundere, to pour, giving diffundere, from which English takes diffuse.

The subject of the blank is it, third person singular, so the present-tense verb takes a plain -s. Diffuse plus -s gives diffuses.

Why the others are wrong

• (a)
  difusez — difusez fails twice over — a single f where the word needs two, and a z ending that belongs to no English third-person verb form.
• (b)
  difuses — difuses takes the right ending and then drops an f. Diffuse needs both: one from the assimilated prefix dif-, one from the stem -fuse.
• (d)
  diffuzes — diffuzes gets the double f right and then swaps the s for a z. This verb keeps its s throughout — diffuse, diffuses, diffused, diffusion.

A fill-in-the-blank spelling item gives you a sentence, but the sentence rarely decides anything — all four options mean the same and fit the same slot. What decides it is orthography, and two features are being tested at once here.

Double f: diffuse descends from Latin diffundere, where dis- assimilates to dif- before an f.

s, not z: the s survives through diffuse, diffused and diffusion. This is a stem s, so it is untouched by the British and American quarrel over -ise and -ize endings.

Key facts

• Diffuse is spelt with a double f and an s: diffuse, diffuses, diffused, diffusion.
• The double f comes from Latin dis- assimilating to dif- before fundere, meaning to pour.
• A present-tense verb with a third-person singular subject such as it takes -s.

Common traps

• Hearing the z sound at the end and spelling the word with a z
• Cutting one f because the word is pronounced with a single f sound
• Trying to reason from the meaning of the sentence, which every option satisfies equally

No directly related past PYQ was found.