Read the paragraph from the introduction [paragraphs 1-3]. If the keystone species disappears from the ecosystem, no other species can do its job. The ecosystem would have to radically change, allowing new and possibly invasive species to take over the habitat.
Which of the following words, if it replaced "radically" in the second sentence above, would CHANGE the meaning of the sentence?
A. thoroughly
B. completely
C. drastically
D. beneficially

b. b. b. is the anoswer

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.