Chất tinh khiết là: A. Có tính chất thay đổi

B. Có lẫn thêm vài chất khác

C. Gồm những phân tử đồng dạng

D. Không lẫn tạp chất

The FitnessGram™ Pacer Test is a multistage aerobic capacity test that progressively gets more difficult as it continues. The 20 meter pacer test will begin in 30 seconds. Line up at the start. The running speed starts slowly, but gets faster each minute after you hear this signal. [beep] A single lap should be completed each time you hear this sound. [ding] Remember to run in a straight line, and run as long as possible. The second time you fail to complete a lap before the sound, your test is over.

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c. heat could easily travel through the molecules in the air and escape/enter the flask. removing the air slows down the heat transfer.  

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explanation: