Directions: Choose the correct form of verb

My legs are aching now because I ____ since six o clock.

The present perfect continuous tense (have/has + been + verb-ing) shows an action starting in the past and continuing to the present. 'Have been swimming' is correct because the swimming caused current aching. 'Have been swim' is grammatically wrong, 'swam' is simple past (doesn't show duration), and 'swimmed' is not a valid word.