一茶杯放在桌面上，下列关于茶杯和桌面的受力论述中，正确的是（   ）

“Feng Robert wanted to do well in his exams．He spent all his free time studying．He wanted to be a scientist when he grew up．Unluckily，this meant that

he was almost always serious． Though he was young，he didn't relax and enjoy himself．

    Robert's first important exam was coming soon．He had spent almost all the week preparing for it．He had got little sleep and felt very tired．However, Robert thought that he was ready for the exam．“I worked harder man anyone else．I’m  ▲ I will get a good grade，”he thought during the breakfast．

    After breakfast．he started to feel a little ill．He also had a little headache and found it difficult to concentrate (专·心)．But he refused to believe that he'd fail(不及格)the exam．Robert arrived at me school hall and took his seat． He noticed how relaxed and happy his mends looked．

    “They are just underprepared (准备不充分的)，”he thought as the exam began．

     Unluckily, Robert wasn't able to finish his exam．Twenty minutes after the exam started, he felt very hot and uncomfortable．He then s，“slumped(瘫倒) in his chair and one of his friends called for help．His teacher took him to the school hospital at once．In the hospital，the doctor told him that he was too tired to finish the exam．

    He spent the lunch break in the nurse's office．He looked outside and watched his friends．They were playing happily．Robert decided that from then on, he wouldn't take it all so seriously  .

“A balanced (平衡的)life is more important．I guess I know what to do next time，”he thought．

在“用天平称物体质量”的实验中，张强同学用已调节好的天平在称物体质量时，通过增、减砝码后指针偏在分度盘中线左边一点，这时应该（   ）