下列物质不属于醛类的物质是（　　）

Can you tell _______ the way to the ocean? （  ）

                                                                                        Everyone (人人)needs to be healthy.
   Do you eat healthy food every day? It’s important (重要)for your health. You need fruit, like oranges, apples and bananas. “ An apple a day can keep away（避开） the doctor (医生).” You also need vegetables, like carrots and broccoli. Fruit and vegetables are good for our health. But don’t eat lots of chocolate. It’s not good healthy food. What about dessert? Don’t eat lots of dessert because it’s not good for your health. Many people eat lots of chocolate and dessert, so they are unhealthy. Sports can also keep you healthy. Play sports every day. Don’t be lazy. You can be healthy.

命题“$\forall x\in \mathit{R}$,$x^2\ge 0$”的否定为(     )

二元一次方程组 $\{\begin{array}{c}x\text{+y=21}\\ 3x-2y=8\end{array}$ 的解为____