下列核反应方程中，属于$\alpha$衰变的是（）

A．	${}_7{}^{14}N+{}_2{}^{4}He\to {}_8{}^{17}O+{}_1{}^{1}H$	B．	${}_{92}{}^{238}U\to {}_{90}{}^{234}Th+{}_2{}^{4}He$
C．	${}_1{}^{2}H+{}_1{}^{3}H\to {}_2{}^{4}He+{}_0{}^{1}n$	D．	${}_{90}{}^{234}Th\to {}_{91}{}^{234}Pa+{}_{-1}{}^{0}e$