If t (tall) is dominant to t (short), then what proportion of the progeny will be tall in the following cross: tt × tt?

If you use the Punnett Square to figure out the proportion, then you will find that there are two TT and two Tt. Therefore, both of them have the dominant gene, but only 50% carry and have the chance of the recessive gene. 

answer: - b cells

explanation: -

there are many types of white blood cells present in our body. one such class of white blood cells are the lymphocytes. the b cells belong to the lymphocyte class of wbcs. the b cells perform their action against the pathogen by producing specialised proteins called as antibodies. these antibodies are produced as and when the b cells encounter pathogens in response to the antigens present on the surface of the pathogen. these antibodies bind to the antigens present on the pathogen and then, target them for destruction by other cells of the immune system.

so, the b cells mediate their action by identifying the antigens present on the surface of the pathogens and then producing antibodies in return to mediate its effect.