Aluminum isn’t found free in nature, owing to its always ____ with other elements, most commonly with oxygen.
________the cost, that will be very little.
The fact has worried many scientists ________the earth is becoming warmer and warmer these years.
The number of foreign students attending Chinese universities ________rising steadily since 2010.
Only then ________how much damage had been caused.
Memories ________on him as soon as he met the friend whom he had not heard from for over thirty years.