The N $\mathrm { N } - \mathrm { H } \text { bond in the ammonium ion, } { } ^ { + } \mathrm { NH } _ { 4 }$

The N $\mathrm { N } - \mathrm { H } \text { bond in the ammonium ion, } { } ^ { + } \mathrm { NH } _ { 4 }$ , is formed by the overlap of what two orbitals?

A) sp³-sp³
B) sp³-sp²
C) sp²-sp²
D) sp²-s
E) sp³-s