根据所提供的信息, 选择正确的答案。

Piano Player Wanted

Boys and girls, are you music fans? Can you sing or dance? Can you play the piano?

Welcome to our Sky Rock Band.

Please call Victor at 6891-3453 or give an email to music@yahoo. com.

                                                                                    Let's Learn Japanese

Can you speak Japanese? Do you want to learn Japanese? Come to the Japanese club（俱乐部）now.

Time: 9:00 a. m. -11:30 a. m. (from Monday to Friday)

Call Henry at 6543-1248.

Ping-pong Club

Do you like to play ping-pong? Do you want to play ping-pong well? Mr. Chen can teach you. You can come here every Friday afternoon.

Tel: 8623-1940

Add: Room 102 in School’s P. E. Building.

                                                                                                Wanted

Can you swim? Do you like children? Can you help them with swimming on weekends? Come here!

Call Jane at 8330-9785 for more information (信息).

由于水土流失严重，黄河成为世界上____ 最大的河流。水土流失不仅使土壤失去了大量养分，而且泥沙淤积在河底，使下游河床不断抬高并高出地面，成为“____ ”。

猜想一：琴弦发出声音的音调高低，可能与琴弦的横截面积有关；

猜想二：琴弦发出声音的音调高低，可能与琴弦的长短有关；

猜想三：琴弦发出声音的音调高低，可能与琴弦的材料有关．

为了验证上述猜想是否正确，他们找到了上表所列4种规格的琴弦，因为音调的高低取决于声源振动的频率，于是借来一个能够测量振动频率的仪器进行实验．

①为了验证猜想一，应选用编号为____、____的琴弦进行实验；

②为了验证猜想二，应用编号为____、____的琴弦进行实验；

③表中有的材料规格还没填全，为了验证猜想三，必须知道该材料的横截面积是____mm²。

The man sitting ____ ____ ____ is my best friend.

下列有关有机高分子化合物说法中，正确的是(　　)