【C5】A．beforeB．thatC．soD．until

【C20】A．beforeB．whenC．sinceD．until

【C5】A．andB．howeverC．soD．therefore

【C5】A．whichB．thatC．whatD．than

【C5】A．beforeB．whileC．whenD．after

【C19】A．beforeB．afterC．onD．until

Its such a difficult problem______no one can work it out.A．asB．thatC．soD．this