从 A, B, C, D四个选项中，选出可以填入空白处的最佳答案： 1.       – Jack? -- ____.  A. Present, Sir.    B. I am, Sir   C. Here, Sir   D. Yes, Sir. 2.       ---What are you busy with ? --- We are carrying out a research ____ the causes of cancer. A. into    B. onto   C. to   D. in 3.       ---____ the paper ? --- No, I have still got one page to finish. A. Have you done   B. Do you do   C. Did you do    D. Had y