题目

71. The family had to move to a much smaller house in order to manage __________（以减轻经济负担）. 72. At that moment, the door __________ open（门开了）, and Roy walked in. 73. It was here that I discovered that I was__________ (过敏的) to the tiny flies which bit me and made my face swell. 74. It was a great __________（荣幸）to hear her sing. 75. He was a very __________ (体贴的) boy for someone so young. 76. Her evidence suggests a different __________ (解释)of the events. 77. At the i__________（休息，间隔）, wine was served. 78. The activit若在抛物线y=ax2（a＞0）的上方做一个半径为r的圆与抛物线相切于原点O，且该圆与抛物线没有别的公共点，则r的最大值是？