The lower limit of Class B airspace is ________.

The lower limit of Class B airspace is the Earth's surface. Class B airspace is designed to protect aircraft operating near busy airports and is typically structured around the terminal area of those airports. It extends from the surface up to a specified upper limit, which varies depending on the airspace system and local requirements.

The ground-level designation ensures that all aircraft operating within this highly regulated airspace are given the necessary separation and control, increasing safety in environments where both commercial and general aviation flights may be taking off and landing. The surface designation also allows for optimal management of air traffic, especially in busy metropolitan areas.

In contrast, the other options represent altitudes above the surface that do not apply to the lower limit of Class B airspace. Class B operates from the ground up, signifying its importance in managing air traffic in densely populated flight areas.