Answer:
The problem in the sentence is the following one:
"Thirsty", which is an adjective, should be changed to "thirstily", which is an adverb of manner.
Step-by-step explanation:
The sentence would look like this:
Ruth was parched, and she thirstily drank all the remaining water in the canteen.
Since Ruth was parched, she drank all the remaining water in the canteen in a thirsty way, that is, thirstily.
The adjective "thirsty" became the adverb of manner "thirstily" with the addition of the suffix "ly". Since "thirsty" ends in letter "y", preceded by a consonant ("t"), "y" should be replaced with "i", and only then the suffix can be added.