Sentence improvement – choose the correct phrasal verb and pronoun placement for dictionary consultation: replace 'look for them in the dictionary' with the more idiomatic 'to look them up in the dictionary' while keeping agreement with the plural noun 'words'.

Given sentence
'Whenever my students come across new words, I ask them to look for them in the dictionary.'

Concept/Approach
For consulting a reference source like a dictionary, the standard phrasal verb is look up. With pronouns, particle placement prefers 'look them up' (not 'look up them'). Also, the pronoun must match the plural antecedent 'words'.

Step-by-step correction
1) Replace 'look for … in the dictionary' with the precise phrasal verb: 'look … up'.2) Keep plurality: 'them' refers to 'new words'.3) Final: '… I ask them to look them up in the dictionary.'

Why other options are incorrect
'to look it up' → pronoun number mismatch ('words' is plural).'to look at them' → means 'to view', not 'to check meaning'.'No improvement' → retains a wordier, less idiomatic phrasing.

Final Answer
to look them up