  Assume that A is a unital separable simple C*-algebra with real rank zero,
stable rank one, strict comparison of projections, and that its tracial simplex
T(A) has a finite number of extremal points.
  We prove that every self-adjoint element a in A in the kernel of all tracial
states is the sum of two commutators in A and that every positive element of A
is a linear combination of projections with positive coefficients.
  Assume that A is as above but \sigma-unital. Then an element (resp. a
positive element) a of A is a linear combination (resp. a linear combination
with positive coefficients) of projections if and only if for all \tau in T(A),
the extension \bar\tau to the enveloping von Neumann algebra has a finite value
for the range projection of a.
  Assume that A is unital and as above but T(A) has infinitely many extremal
points. Then A is not the linear span of its projections. This result settles
two open problems of Marcoux.
