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In the process of scientific inquiry, a theory is best defined as:

The absorption of longwave radiation by certain gases in the lower atmosphere is responsible for:

Which of the following would be true if Earth did not have an inclined axis?

Most of the radiation emitted by the earth and its atmosphere is in the category of:

The atmosphere is heated chiefly by radiation emitted from the earth’s surface.

Which of the following spheres is composed exclusively of water?

C3 convertase cleaves C3 in blood plasma, leading to a conformational change in the C3b fragment that exposes its reactive thioester group. The activated C3b is potentially harmful to the host, if it becomes covalently attached to a host cell, rather than to the surface of a pathogen. This deleterious outcome is largely avoided by:

Defects in C3 activation leads to impaired tagging of pathogens.

The difference between activating and inhibitory receptors  in NK cells lie in their association with accessory proteins that promote downstream signaling, or in their ability to recruit and activate inhibitory phosphatases, respectively.  

Mucosal surfaces are found in tissues such as the gastrointestinal tract, the reproductive tract and the mouth and respiratory tract. While the mouth and respiratory tract are routes of virus but not bacterial infections, the gastrointestinal tract is the route for bacterial but not virus infections.