I have read the answer given to Ralph. At the very least this is a VERY bad example to give in a lesson. The answer demonstrates that either tense could be used depending on the message the writer wishes to convey.
This is precisely what we want to explain in the lesson. You can use either podía or pudo but using one or the other will convey a different meaning/message to the listener. Using the time adverb "anoche" actually illustrates how even with a word that "generally" triggers a tense, we can actually use a different tense if the strengh of the message is on the meaning "succeced + something seen as completed" or "wasn't being able to + ongoing action".
In B2/C1 lessons you will find examples and questions that are more complex than what you find in lower levels. The language becomes more complicated with different structures, meanings, expressions, exceptions, nuances in an advance level.
Un saludo cordial
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