Which of the following is true about the determination of the master variable in symbolic operation? ( )

A、If the master variable is not specified, MATLAB will take the lowercase letter nearest to x in alphabetical order except for i and j as the master variable.;
B、When there is x in a symbolic expression and the expression specifies no master variable, the system's default master variable is x.;
C、If there are multiple variables in the symbolic expression s, the function symvar(s) will return a system's default master variable.;
D、If there is no x in a symbol expression, but the expression includes y and z, then MATLAB will take z as the master variable by defualt.