  Based on the first-principles calculations, we recover the silent topological
nature of Cd3As2, a well known semiconductor with high carrier mobility. We
find that it is a symmetry-protected topological semimetal with a single pair
of three-dimensional (3D) Dirac points in the bulk and non-trivial Fermi arcs
on the surfaces. It can be driven into a topological insulator and a Weyl
semi-metal state by symmetry breaking, or into a quantum spin Hall insulator
with gap more than 100meV by reducing dimensionality. We propose that the 3D
Dirac cones in the bulk of Cd3As2 can support sizable linear quantum
magnetoresistance even up to room temperature.
