Read this sentence:
Students gave apples to their teachers in the old days. This was a sign of appreciation.
How can this sentence be rewritten so it is clearer?
A. As a sign of appreciation, students gave apples to their teachers in the old days.
B. Because students gave apples to their teachers in the old days, it was a sign of appreciation.
C. Students gave apples to their teachers in the old days, and this was a sign of appreciation.

the present continuous is a verb tense which is mainly used to indicates that an action is in progress at the moment of speaking.  

it is formed using: person + verb to be (am/is/are) +verb + - ing form  

example: i am studying at this moment .

in addition, the present continuous is also used in order to express things that will or will not happen in the near future.

example: i am having dinner with some friends tonight.

the speaker sometimes   uses it   to express that an action is repeated and it causes irritation on the part of the speaker

example: my sister is constantly complaining about everything.

a. energy is conserved on earth, but matter is not.