5. He was as ______ as a donkey. Even though I had tried very hard, he did not change his mind. (A) efficient (B) diligent (C) outstanding (D) stubborn

10. The dirty air, known as smog, is a combination of weather conditions, fumes and smoke from cars and factories and therefore is sure to ______ many serious health problems. (A) bring up (B) give rise to (C) look forward to (D) have little to do with