19. When it is struck, a tuning fork produces an almost pure tone, retaining its pitch over a long period of time.

　　音叉被敲击时，产生几乎纯质的音调，其音量经久不衰。

　　20. Although pecans are most plentiful in the southeastern part of the United States, they are found as far north as Ohio and Illinois .

　　虽然美洲山河桃树最集中于美国的东南部但是在北至俄亥俄州及伊利诺斯州也能看见它们