下列各句中，标点符号使用正确的一句是（    ）

课文填空

    The skin is ____ essential part of your body and its ____ organ. You have three ____ of skin which act as a barrier ____ disease, poisons and the sun's harmful rays. The ____ of your skin are also very ____: it keeps you warm or cool; it prevents your body from ____ too much water; it is where you feel cold, heat or pain it gives you your sense of ____. So you can imagine, if your skin gets ____ it can be very serious. First aid is a very important first step in the ____ of burns.

烧瓶C中发生反应如下：

Na₂S（aq）+H₂O（l）+SO₂（g）═Na₂SO₃（aq）+H₂S（aq）   （Ⅰ）

2H₂S（aq）+SO₂（g）═3S（s）+2H₂O（l）            　（Ⅱ）

S（s）+Na₂SO₃（aq） $\underset{\_}{\underset{\_}{△}}$ Na₂S₂O₃（aq）               （Ⅲ）

计算：（﹣12）+（+$\frac{6}{5}$）+（﹣8）+（﹣$\frac{7}{10}$）+（﹣$\frac{1}{2}$）=（　　）