请根据左边所给的单词以正确的形式填空,

请根据左边所给的单词以正确的形式填空,
46.lock I lost my keys and couldn’t ___ the box.47.experience Mary was already an ___ primary school teacher at the age of 24.48.threat Their lives were frequently ___ by gas explosion.49.perform The first American ___ of this opera was in 1926.50.courage It was ___ of Mr.Stone to try and save the drowning man.51.hope He was disappointed so often that he became ___.52.cruel ___ to children or animals is a crime in many countries.53.understanding John and Mary had a ___,but they have made it up now.54.large The factory is being ___ to make room for more machinery.55.eager She could not conceal her ___ for praise.