The immune system tries to stop invasions by foreign agents by either killing cells directly or by

a. creating an unstable environment.
b. erecting mucous membranes.
c. reallocating critical functions.
d. flushing the foreign bodies out.

Why are radio observatories located in valleys far away from urban centers? a. to shield them from electromagnetic interference b. to avoid sound pollution c. to avoid disturbing people living in the city d. to shield them from water vapor