(1) A person is guilty of assault in the first degree when:
(a) He intentionally causes serious physical injury to another person by means of a deadly weapon or a dangerous instrument; or
(b) Under circumstances manifesting extreme indifference to the value of human life he wantonly engages in conduct which creates a grave risk of death to another and thereby causes serious physical injury to another person.
(2) Assault in the first degree is a Class B felony.