题目

 — Catherine, I have cleaned the room for you. — Thanks. You ______ it. I could manage it myself. A. needn’t do               B. needn’t have done C. mustn’t do               D. shouldn’t have done 答案：B二次函数y=ax2+bx+c的图象与x轴交于B、C两点，与y轴交于A点．（1）根据图象确定a、b、c的符号，并说明理由；（2）如果点A的坐标为（0，-3），∠ABC=45°，∠ACB=60°，求这个二次函数的解析式．