Questions

Primary key helps us to identify a row uniquely in a relation

A Skeleton is what is whole but incomplete, have no eyes, yet can see. We can see, and see right through it. The largest part is one fourth of what it once was i.e body.

The gram formula mass is nothing but molar mass.

Composition :

Symbol      Element       Atomic weight       Atoms
   

     P         Phosphorus       30.973762               1       = 30.973
   

     O         Oxygen             15.9994                  4       =  63.998
   

     N         Nitrogen            14.0067                  3       =  42.020
   

     H         Hydrogen          1.00794                  12      =  12.096

Total   =  30.973+63.998+42.020+12.096 = 149.087

Fear of places and situations that might cause panic, helplessness or embarrassment is called Agoraphobia. 

Fear of crossing road causes some panic in them and comes under Agoraphobia.

A greenhouse gas is a gas in an atmosphere that absorbs and emits radiant energy within the thermal infrared range. This process is the fundamental cause of the greenhouse effect. 

Carbon dioxide is a main greenhouse gas, found in car exhaust pipes.
Methane and nitrous oxide is another type of greenhouse gas.

Oxygen is the only clean gas out there because we need it to survive. Therefore, the only gas that is not a greenhouse gas is Oxygen.