Free SAT Practice Question

Ornithologist Junko Aritomo has spent several seasons recording the songs of warblers in a densely forested valley. There the males sing an unusually slow song whose long, clear notes carry through the thick foliage to females perched far off. ______ the slow pace leaves wide gaps between the notes, and in those gaps a singing male can hear whether a rival is answering from somewhere nearby.

Which choice completes the text with the most logical transition?