Answer:
His mistake was that he also gave the plants in the music room some fertilizer
Explanation:
When carrying out a reliable experiment, the only variable should be the question being tested. The question here is whether playing music for plants helps them grow faster. Therefore, all the plants should be grown in exactly the same conditions apart from the presence or absence of music.
However, Jo also added fertilizer to the plants in the room where music was playing. Therefore, we don't know whether the fertilizer or the music caused the plants to grow faster.
Instead, Jo should have not given fertilizer to any of the plants, or given fertilizer to all the plants.
