Connection between AP and client card

Given an AP and a wireless client card,

if you separate them slowly, at certain distance the signal is so small that
they stop communicating each other.

That could be for one of these four reasons:

- Insufficient sensibility in the client card
- Insufficient power in the client card
- Insufficient sensibility in the AP
- Insufficient power in the AP

a) What is the reason that most often make them stop communicating?

b) What is the best solution to keep them connected at that distance?
Putting an antenna in the client card? Putting the antenna in the AP?


Two more questions:

Given a, for example, 5 dBi isotropic antenna,

c) It increases the sensibility of the device where it is put? the
efficiency with which the power is radiated? both?
d) If the range without the 5 dBi antenna is, let's say, 100 meters, what
range could we achieve putting the antenna?


Thanks,


Josechu