【B15】A．showsB．monitorsC．informsD．instructs

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

【B15】A．whenB．asC．ifD．though

【B15】A．WhichB．WhoC．WhenD．What

【B15】A．takeB．flyC．findD．make

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

【B15】A．byB．forC．uponD．from