Answer:
3. They may not have time to read the whole thing.
Step-by-step explanation:
1: You cannot gain a general understanding by skimming a text.
2: It doesn't make sense to skim a text if you need help defining a word, because part of defining a word is looking for context clues.
4: If they are looking for specific information they would have to analyze the text instead of quickly looking over it.
Therefore 3 is the right answer. Hope this helps :)