Despite Often Living in the Bladder, Why Is the Blood

Despite often living in the bladder, why is the blood fluke, Schistosoma, not easily washed out of its host?

A) They secrete a glue-like substance that attaches them firmly to the walls of the tissues they colonise.
B) Blood flukes grow a rhizoid that secures them to the host tissue.
C) They secrete a protease enzyme that partially degrades host wall tissue, after which the abaxial axis of the fluke effectively 'fuses' with the degraded host tissue, permanently anchoring the fluke in place.
D) They live in blood vessels in the tissue walls.
E) They have a posterior with hooks and barbs.