In this task, you are given a context, a subject, a relation, and many options. Based on the context, from the options select the object entity that has the given relation with the subject. Answer with text (not indexes).

Context: Marchantiales is an order of thallose liverworts that includes species like "Marchantia polymorpha", a widespread plant often found beside rivers, and "Lunularia cruciata", a common and often troublesome weed in moist, temperate gardens and greenhouses., Riccia fluitans , whose common name is floating crystalwort , is an aquatic floating plant of the liverwort genus Riccia which is popular among aquarists as a retreat for young fry and is used in live - bearing tanks . It can be found floating in ponds , and often forms thick mats on and under the water surface . It normally grows quickly at the surface . When kept about two to three inches below an ordinary fluorescent bulb or in a pond exposed to full sunlight , it will form dense , bright green mats . Any single branch or antler bud can reproduce into a large colony if the plant is kept in proper conditions . It normally floats , but can also be attached to underwater objects such as logs and rocks . It can be attached with plastic mesh . Floating crystalwort is generally not compatible with duckweeds , as they cover the surface of the water quite rapidly , crowding the Riccia out . It is also easily overtaken by hair algae ., Riccia is a genus of liverworts in the order Marchantiales., Subject: riccia fluitans, Relation: taxon_rank, Options: (A) genus (B) order (C) species
species