What format is given to variable$-$length texts so that transformers can analyze them and generate the Z representation? a$.$ The entire text is embedded into a vector of length equal to the longest instance, with shorter instances padded with zeros.b$.$ One lexical component at a time is introduced, and the processing accumulates in the transformer's internal state.c$.$ The code of each lexical component is an entry in matrix X$.$d$.$ The texts are of fixed size but very large, as they only apply to complete books or large volumes of text extracted from the web, so it doesn't matter if they are truncated at the end.