Answer: the error is "We also had a 20 metre pool."
Explanation: In grammar, tense is a category that expresses time reference with reference to the moment of speaking. Tenses are usually manifested by the use of specific forms of verbs, particularly in their conjugation patterns. In English, tenses include past, present and future. When writing we need to make sure that be consistent in the use of tense. In the given extract all the information is in present (we have, we run, etc) except for the phrase "We also had a 20 metre pool" which is in past, it should be "We also have a 20 metre pool."
I think the answer is B. Forced vibrations, such as those between a turning fork and a large cabinet surface, result in a much lower sound than was produced by the original vibrating body.
