语法填空

    ____ is well known, optimism(乐观)plays an important role in one's life. Although we are facing many difficulties, we should keep optimistic. Optimism is a mental(精神的)attitude or world view and it means expecting ____ best possible result from any given situation.

    I did ____ (poor) in English in the past, but I never gave up struggling(搏斗). All my families, teachers and friends encouraged ____ (I) and they taught me a lot of ____ (use) ways of learning English. I ____ (keep) practicing as they told me to do. I listened to English news and songs and I read interesting English stories. At weekends, watching some English movies was another good way to make my English better. ____difficulties I came across, I tried my best to work them out. I believe in myself, ____ (believe) that hard work would finally pay off. Gradually, I make progress in my English. Nowadays, I can remember English words ____ (easy) and use them freely when communicating with foreigners.

    Thanks to optimism, I have become one of the top English ____ (learn) in my school now.

如果代数式$\frac{4}{\sqrt{x-3}}$有意义，则x的取值范围是(     )．

质点做直线运动的位移x与时间t的关系为$x=5t+t^2$(各物理量均采用国际单位制单位)，则该质点(　　)