Why are gamma ray observatories difficult to construct on earth's surface?

a) objects in the universe do not emit gamma rays.
b) gamma rays cannot be detected by our current technology.
c) gamma rays have the lowest energy on the electromagnetic spectrum.
d) gamma rays do not penetrate through earth's atmosphere to reach its surface.

What tightening torque should be used for a hexagonal head screw (not split-bolt) on a 250 kcmil conductor?

The field between two charged parallel plates is kept constant. if the two plates are brought closer together, the potential difference between the two plates either a) decrease b) does not change c) increase?

Acommercial refrigerator with refrigerant-134a as the working fluid is used to keep the refrigerated space at -35°c by rejecting waste heat to cooling water that enters the condenser at 18°c at a rate of 0.25 kg/s and leaves at 26°c. the refrigerant enters the condenser at 1.2 mpa and 50°c. if the compressor consumes 3.3 kw of power, determine (a) the mass flow rate of the refrigerant, (b) the refrigeration load, (c) the cop, and (d) the minimum power input to the compressor for the same refrigeration load.