【B15】A．observedB．examinesC．studyD．questioned

I（)English for about ten years.A、have studiedB、studiedC、studyD、have study

【B15】A．fillingB．blockingC．fuelingD．contributing

【B15】A．whenB．asC．ifD．though

【C13】A．observedB．observableC．detectedD．detectible

【C7】A．studyingB．studiedC．studyD．studies

【C4】A．attendB．participateC．studyD．belong