题目

If you don’t understand something, you may research, study, and talk to other people _______ you figure it out.        A. because               B. though                  C. until                      D. since 答案：【答案】C 【解析】考查连词。句意：如果你无法理解某个东西，你可能会研究，学习和他人探讨直到你解决为止。根据句意可知选C。方程x2-4=0的解是（ ）A．x1=2，x2=-2B．x=-2C．D．x=2