Aluminum isn’t found free in nature, owing to its always ____ with other elements, most commonly with oxygen.
31. Lewis was much admired ______ his work on the advanced medical research.
30. The pilot, whether ______ or design, made the plane do a sharp turn.
29. If you ______ go, at least wait until the rain stops.
28. The reason for his success is ______ he worked very hard.
27. He couldn’t ______ the fact that the money was found in his house.