特殊情况下无《辐射安全许可证》可订购放射性核素及其标记化合物。
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Since heat will not flow freely from a body at a () temperature to another at a () temperature, it is necessary to expend mechanical work, heat, or electrical energy from an external source to achieve refrigeration.
A. lower;higher
B. shallower;deeper
C. larger;smaller
D. higher;lower