Online shopping is increasing popular among people in recent years. Some people liked to buy goods online. And I don’t like it, because online shopping is more reliable than the traditional sale. Firstly, some dishonest merchant offer customers to false information online. Secondly, consumers can’t feel the items that is usually presented by means of pictures. This means the items delivering may differ from which consumers expect. As a result, they may risk returning the items at their own expense. Thirdly, the items might be damaged during delivery. In word, online shopping isn’t reliable at all.

对装入球磨机的钢球直径有何要求？