As my son . . . . . . . . down by the river, I . . . . . . . . his room thoroughly. A. fishes/have cleaned B. is fishing/cleaned C. has fished/clean D. fished/have been cleaning E. was fishing/cleaned

The correct answer is: As my son fishes down by the river, I have cleaned his room thoroughly.

The other options are incorrect because they do not correctly describe the relationship between the two actions.

• Option B, “is fishing/cleaned,” is incorrect because it implies that the son is both fishing and cleaning his room at the same time. This is not possible, as the two actions require the son to be in different places.
• Option C, “has fished/clean,” is incorrect because it implies that the son has already finished fishing and is now cleaning his room. However, the sentence states that the son is still fishing, so this cannot be the case.
• Option D, “fished/have been cleaning,” is incorrect because it implies that the son has finished fishing and has been cleaning his room for some time. However, the sentence states that the son is still fishing, so this cannot be the case.
• Option E, “was fishing/cleaned,” is incorrect because it implies that the son was fishing and then cleaned his room. However, the sentence states that the son is still fishing, so this cannot be the case.

Therefore, the correct answer is: As my son fishes down by the river, I have cleaned his room thoroughly.