Correct Answer:
B. Since
Correct answer: Since. “Since” is used with a point in time, such as half past nine.
Background: Present perfect continuous: has been + verb-ing.
- For: “For” is used with a length of time, such as two hours.
- From: “From” does not fit this present perfect continuous sentence.
- At: “At” does not fit this present perfect continuous sentence.
Did you know? Use “since” for a starting point and “for” for a duration.
Memory trick: Since = point; For = period.