1.There is no denying that you ___________(越仔细越好) in dealing with this matter.
　　解析：can never be too careful / can not be too careful

　　2. Only when I reached my thirties __________________________ (我才意识到读书是不能被忽视的)
　　解析：did I realize that reading cannot be neglected
　　did I realize that reading is unignorable

　　3. Much ___________________ (使研究人员感到惊讶),the outcome of the experiment was far better than they had expected.
　　解析：to the researchers’ surprise

　　4. Oh, my, I can’t find my key; __________________________(我一定是把它放在哪儿了).
　　解析：I must have left / put it somewhere

　　5.I ________________________(宁愿加入你们去做义工)than go to the beach for a holiday.
　　解析：would rather join you as a volunteer