How to tell if chemical reaction is acid-base, precipitation, redox, or gas-forming

What type of a reaction are these:

Use "redox", "precipitation", "acid-base", and "gas-forming" but use each only once.

(a) Cu (s) + 4NHNO3 (aq) –> Cu(NO3)2 (aq) + 2H2O (l) + 2NO2 (g)
(b) Cu(NO3)2 (aq) + 2NaOH (aq) -> Cu(OH)2 (s) + 2NaNO3 (aq)
(c) CuO (s) + H2SO4 (aq) -> CuSO4 (aq) + H2O (l)
(d 3CuSO4 (aq) + 2Al (s) -> 3Cu (s) + Al2(SO4)3 (aq)

Thanks in advance for any help!

a) Gas forming (NO2(g) is formed) This is also a Redox (Cu(s) is forming +2 ion Cu2+)
b) precipitation (2 soluble compounds form an insoluble solid)
c) Acid-base (H2SO4 is an acid, CuO is a base, and you form H2O(l))
d) Redox (Cu2+ ions form Cu(s) and Al(s) form Al3+ ions)