Why the recombinant DNA is reintroduced into the bacterium?

Bacterial cells can pick up the DNA through the process of transformation. Lambda phages are used by eliminating the middle of its liner genome and adding the foreign DNA in the created space. The phage is then introduced into the bacterial cell where it replicated itself via the lysogenic cycle.

It would be (at) + cu (s)

Only one reactant yields 2 or more products. meaning you will only have 1 product and it will break down into 2 or more products. eg. h2o -> h2 + o2