Sherlock Holmes was a world-famous detective who used observation skills to solve crimes. Which piece of evidence from The Mystery of the Lost Gem best supports this topic sentence?
a
Sherlock Holmes noticed scratches on the floor near the wall.

b
Sherlock Holmes opened the door, and Mrs. Hudson introduced his client.

c
Sherlock Holmes silently vowed that he would find the gem and the thief.

d
Sherlock Holmes was reading by the fire when he heard a knock at the door.

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.