重力式滚筒输送机运动物品时，当输送硬底物时，最少要（ ）个滚筒支撑物品才能保证正常输送工作，而柔性物品则最少需要（ ）个滚筒才能保证正常的输送。
A. 3，4
B. 4，5
C. 2，3
D. 3，5
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He is the best _______I have met.
________is the world's sixth largest country by total area.
The new student was _________to introduce himself to his classmate.
He was happy with the ________of his welcome
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