Thank you so much for your help. I had tried the W key a bunch of times before posting my initial inquiry, but the clip would come down and land in the audio file rather than in the main (B Roll) portion of the timeline. Now I can see why you block out the storyline in advance.
The strange thing though is that in the past, when working on projects, whenever I dragged down a clip to the timeline, I always had the option of either adding it as a new clip or replacing an old clip with it. The option was simply always there, and when I go to the online tutorials, they also say that the option should always be there. I even tried restarting my computer hoping that the option would return.
But thank you anyway – you have solved my problem for this project!!!!!