Read the following passage and mark the letter A, B, C, or D on your answer sheet to indicate the correct answer to each of the questions.

Becoming a teacher demands not only knowledge in an academic field but also a personal commitment to lifelong learning, and enthusiasm for sharing knowledge with other people. To become one of those noble educators in the USA, one has to satisfy several basic requirements. First and foremost, it is a prerequisite to have bachelor's degree in education. In the event that a candidate already has a bachelor's degree in another field, a teacher preparation program is needed. But that is not all. Almost every school in the USA understands that real classroom teaching experience is a vital part of a teacher's training. Before taking over a class, a person typically needs to complete a training program, including working as a supervised student teacher. People who want to become university teachers need master's degrees. Getting a master's degree is a necessity, but if it is gained too early, there may be concerns that the candidate lacks the real-world experience to go with it. In fact, very few schools want to hire novices with little or no classroom experience and even if they are accepted, theyare usually ill-paid. One wise solution to the issue is for future postgraduates to start working as teachers before going on to gain their master's degree. Besides knowledge and experience, certain personal qualities are also required. A teacher should be positive, prepared, focused, and most importantly, patient. Being a teacher involves being aware of the fact that learning sometimes be hard work, even for the most motivated students. Also, teaching can at times be tiring and frustrating, so teaching candidates have to practice being patient with themselves. In short, as in other careers, teaching requires a combination of qualifications, experience, and personal qualities. Teaching candidates meetin mandatory requirements are always in demand in the USA.

The word "mandatory" in paragraph 2 is closest in meaning to_______.