顶芽生长与侧芽生长的关系是（　　）

    It was the last day of the final exam in a college. On the steps of building, a group of engineering students were waiting for the last exam. On their faces was confidence（自信的）. 
    They knew the exam would be easy. The professor (a teacher at a college) had said they could bring any books or notes during the test.
Right after they came into the classroom. The professor handed out the papers. There were only five questions on it.
    Three hours passed. Then the professor began to collect the papers. The students no longer looked confident. On their faces was nervous expression（表情）. The professor watched the worried faces before him, and then asked, “How many of you have finished all five questions?” No hand was raised. “How many of you have answered four?” Still no hands. “Three? Two?” The students moved restlessly in their seats.    “One, then? Certainly somebody has finished one.” But the class kept silent. The professor said, “That is exactly what I expected. I just want to make you know clearly that even though you have completed four years of engineering. There are still many things about the subject you don't know. These questions you could not answer are common in everyday practice.” Then, smiling, he added, “You will all pass this course, your education has just begun.”
    The years have obscured the name of this professor, but not the lesson he taught.

已知等腰三角形的一个内角是70°，则它的底角为 ____