Select the most appropriate option to substitute the underlined word in the given sentence. Pollution levels have increased so largely that they are almost killing us.

Correct — C. The frame decides this, not the word on its own: increased so ___ that they are almost killing us. A so … that frame needs a word of degree, because the that-clause states the result of that degree.

Largely means mainly, for the most part — it marks which part of a claim holds (largely responsible, largely due to rain), so it cannot intensify increased.

Of the four options only highly belongs to the degree class, so it is the substitution the item wants — option (c).

Why the others are wrong

• (a)
  primarily — Primarily means mainly or first of all — the same sense that makes largely wrong here. Swapping one mainly-word for another leaves the degree slot in so … that still empty.
• (b)
  slowly — Slowly reports rate, not degree, and it fights the result clause: a slow rise does not explain levels that are almost killing us.
• (d)
  chiefly — Chiefly is a second synonym of mainly. It changes nothing, because the sentence still lacks a word saying how far the levels rose.

Two adverb classes are being separated here.

Focusing adverbs — largely, mainly, chiefly, primarily — tell you which part of a statement is true, or which cause dominates.

Degree adverbs — highly, greatly, considerably, enormously — answer how much, and that is the question so … that asks.

SSC substitution items usually turn on exactly this split: the printed word is perfectly good English, but it belongs to the wrong class for the slot it sits in, and two of the options are near-synonyms of it.

Key facts

• Largely means mainly or for the most part: it marks proportion, not the intensity of a change.
• The so + adverb + that frame needs a degree word, because the that-clause names the result of that degree.
• Primarily and chiefly are synonyms of largely in its mainly sense, which is why neither can replace it.

Common traps

• Picking a dictionary synonym of the underlined word when the item wants the wrongly chosen word replaced
• Reading only the underlined word and never the frame it sits in

No directly related past PYQ was found.