Implement f(a, b, c, d) = m(3, 4, 5, 6, 7, 11, 15) as a two-level gate circuit, using a minimum number of gates.

(a) Use OR gates and NOR gates.

(b) Use NOR gates only.

PLEASE don't just solve it, but explain one important thing

I know hot to create k-map but I am confused when to use 1s or 0s.

My teacher solved it and for part a) he used 1s but for part b) he used 0s

I don't understand reasoning behind this,

please explain it