Usually the y-axis stands for dependent variable while the x-axis stands for independent variable. And the curve stands for the relationship between the two variables. The control variable and measurement variable often means the independent variables.
The correct answer is (a.) Its spindle-shaped cells allow it to do smooth muscles. The structure of the cardiac muscles which is spindle-shaped is very important for the heart to withstand the stretching of the muscles during the contractions. Also for its smooth surface which is very important to prevent any possible blood clot during circulation.
(Pp)(Yy) —> PY, Py, pY, py
