单项选择
从每小题A、B、C、D中选出一个能填入句中空白处的最佳答案。
This is 3-D printer. And this is an artificial (人工的) ear. Can you believe that scientists printed this artificial ear by this 3-D printer?
A. a                          B.  an                            C.  the              D.  /
              D.  /
