  This paper is addressed to studying the exact controllability for stochastic
transport equations by two controls: one is a boundary control imposed on the
drift term and the other is an internal control imposed on the diffusion term.
By means of the duality argument, this controllability problem can be reduced
to an observability problem for backward stochastic transport equations, and
the desired observability estimate is obtained by a new global Carleman
estimate. Also, we present some results about the lack of exact
controllability, which show that the action of two controls is necessary. To
some extent, this indicates that the controllability problems for stochastic
PDEs differ from their deterministic counterpart.
