In humans select one:
a. back muscles are strong to maintain erect posture.
b. deep back muscles extend from the vertebrae to the ribs.
c. most of the muscle mass in the lower back is from the spinalis muscle.
d. back muscles are not very strong.
e. back muscles are similar in strength to the back muscles of cattle.

a. back muscles are strong to maintain erect posture.

Explanation:

Human back muscles are categorized as superficial, intermediate and deep muscles. The intrinsic (deep) back muscles of the human back regulate the movement of spine and maintain erect posture of human.  

For example, the erector spinae is the largest muscle of the human back and is present on either side of spine. It consists of iliocostalis muscles (lateral), longissimus muscles (intermediate), and spinalis muscles. Together these muscles serve in extension of the spine and maintain the erect posture of the spine.

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