Good eye. Miser could conceivably be translated as "sad", as in "in a sad state, pitiable" but it is probably better that the key have the book's own vocabulary as much as possible. Industria is certainly the option for "industry", but if the book doesn't use it then I would hesitate to have it in the key. I do think it was correct to have industria in the singular, as it is referring to their collective 'industry' and was not meant to say that they each have their own separate industries. In these cases you can very often just substitute the word as singular if it is singular in Engllish and plural if it is plural in English.
flebile nescio quid queritur lyra, flebile lingua murmurat exanimis, respondent flebile ripae