A small business is tracking the number of client accounts it manages over time.

A horizontal asymptote is a horizontal line that a graph approaches as the input increases. Horizontal lines always have equations in the form y=constant, not x=constant. In this graph, the number of client accounts decreases over time and levels off near 125 accounts. Since the graph approaches 125 on the vertical axis, the horizontal asymptote is y=125. Choices x=125 and x=550 are vertical lines, so they cannot represent horizontal asymptotes. Choice y=550 represents a higher value near the beginning of the graph, not the long-term level. Therefore, the correct equation of the horizontal asymptote is y=125, making the answer B.