A therapist wants to study the effects of yoga and meditation on stress relief. She has 60 volunteers who experience varying levels of stress. Half of the participants will be assigned to practice yoga for one month and the other half will practice meditation. Before the experiment begins, all of the participants will be asked to rate their stress levels on a scale from 0 to 10, with 0 representing "no stress” and 10 representing "highest level of stress.” At the end of one month, the participants will be asked to rate their stress level again. The differences in stress levels will be compared. Which of the following describes a completely randomized design for this experiment?

The volunteers’ names are all placed on equal-sized slips of paper and these slips are put into a hat. The therapist draws 30 slips of paper and the corresponding volunteers are placed into the yoga group. The remaining 30 volunteers are placed in the meditation group. After one month, the stress levels between the two groups are measured and compared.

Volunteers are put together based on stress level in groups of two. For each group of two, a coin is flipped and if it is heads, then the first person is placed in the yoga group, and if it is tails, then the first person is placed in the meditation group. The other person will be placed in the other group. After one month, the stress levels between the two participants are measured and compared.

The volunteers’ names are all placed on equal-sized slips of paper and these slips are put into a hat. After the hat is shaken, a slip of paper is drawn without replacement and the corresponding volunteer is placed in the yoga group. This is repeated until the yoga group has 30 volunteers. The remaining 30 volunteers are placed in the meditation group. After one month, the stress levels between the two groups are measured and compared.

The volunteers are grouped based on their current stress level. The 30 volunteers with the highest level of reported stress are placed in one group and the remaining 30 volunteers are placed in another group. The therapist flips a coin and if it lands on heads, then the group with the highest stress levels will receive the yoga treatment. If it lands on tails, then this group will receive the meditation treatment. After one month, the stress levels will be compared between the two groups.