What is the isoelectric point of the following tetrapeptide that contains the amino acids represented by the single letters "ELYK", such that $E$ is at the amino$-$terminal end of the peptide and $K$ is at the carboy$-$terminal end. a$.,6\mathrm{.}60$ b$.6\mathrm{.}69$ ec$.,6\mathrm{.}969\mathrm{.}674\mathrm{.}2510\mathrm{.}07,2\mathrm{.}18$ d$.8\mathrm{.}37($e$.)9\mathrm{.}87$