<p>A must be changed because the way it is stated must be followed by the thing performing the action. “Not carefully watching the clock, there” should be “Not carefully watching the clock, someone …” Because right now it makes no sense what or who is not carefully watching the clock.</p>
<p>i think A appears to be a dangling modifer, not carefully watching suggests the teacher, but the subject in the following sentence is a THERE BE structure, which seems wrong.</p>