Let D be the solid bounded by the sphere x^2 + y^2 + z^2 = 1 and the planes y = x, x = 0, z = 0 in the first octant. Express the volume of D as an iterated integral in terms of spherical coordinates. Do not evaluate the integral.

I am having a really hard time understanding the bounds for phi and theta. An explanation with diagrams would really be helpful. Thank you in advance.