____________caused the plane crash remains unknown.
A. Why
B. How
C. When
D. What
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___________that China has the best and brightest scientists to solve the problems is not just good policy, but something personal.
A. Ensure
B. Ensuring
C. Ensured
D. To ensure
You become more ____________in prison than you had been before, ________ regularly.
A. discipline, study
B. disciplining, studying
C. disciplined, studying
D. disciplining, studied
No country can really develop __________its citizens are educated.
A. if
B. when
C. because
D. unless
Politics are off limits but some topics are __________conversations between us.
A. appropriate for
B. ready for
C. care for
D. hungry for