The Japanese Macaca monkey has been studied in the wild for over 50 years. In 1952, on one small Japanese island, scientists dropped some 1  in the dirt for the monkeys. The monkeys liked their taste, but they found the dirt  2.
   One clever 18-month-old monkey found she could 3 the problem by washing the sweet potatoes in a nearby river. She taught this to her mother. Her 4 also learned this new way and they taught their mothers, too.
   All the younger monkeys 5 learned to wash the dirty sweet potatoes to make them 6 to eat. But many old monkeys found it very hard to learn this and still ate the 7 sweet potatoes.
   Then something very surprising 8. In the autumn of 1958, scientists found that the monkeys on other nearby 9 began washing their sweet potatoes too. Scientists still don’t fully understand how this knowledge was 10 from one island to another.

某白色固体中可能含有氯化钠、碳酸钠、硫酸钠、碳酸钡、氯化钡中的几种物质，为了鉴定该白色固体的组成，拓展性学习小组的同学们进行可如下探究实验：

Ⅰ．称取15克固体于烧杯中，加入足量水充分溶解，过滤得到白色沉淀4.3克和无色溶液．

Ⅱ．Ⅰ中所得4.3克白色沉淀中加入足量稀盐酸，沉淀部分溶解并有无色气体产生，测得剩余沉淀质量为2.33克．

Ⅲ．把Ⅰ中所得滤液分成两份，第一份溶液中加入足量的稀硫酸，既没有产生气泡，也没有产生沉淀，第二溶液中加入足量的氯化钡溶液，没有产生沉淀．

根据上述实验，回答下列问题：

在用单摆测 定重力加速度时，某同学用同一套实验装置，用同样的步骤进行实验，但所测得的重力加速度总是偏大，其原因可能是(　　)