Which Example Below Is Not a Random Assignment to Experimental

Which example below is not a random assignment to experimental groups?

A) Assigning numbers to subjects, put corresponding numbers in a hat and pull out numbers placing the first subject in the experimental group and the second subject in the control group and continue until all numbers are used.
B) Ask subjects whether they prefer to be in the experimental group or the control group, then assign subjects based on their preference.
C) Flip a coin assigning subjects to the control group and to the experimental group.
D) Use a random number generator after assigning numbers to your subjects.