A.I. Explanation:

Option 1: "Keeping the back still makes the muscles tense up and then contract"

Explanation: The text describes how immobility due to back pain leads to muscle tension and spasms, creating a vicious circle of worsening pain. This directly aligns with the option, as it captures the essence of the cycle where keeping the back still results in increased tension and contraction of the muscles.

Option 2: "Strain on any limb resolves itself after some time"

Explanation: While the text mentions that strains generally resolve themselves, it specifically refers to limbs and does not connect to the vicious circle of back pain. This option is too broad and does not address the specific issue of back pain discussed in the text.

Option 3: "Counterproductive exercises are developing pain into such a unique menace"

Explanation: This option misinterprets the text. The speaker discusses immobility and its effects on back pain, not exercises. Therefore, it does not relate to the vicious circle described, making it an incorrect choice.

Option 4: "Extreme ankle pain calls for bed rest"

Explanation: This option is unrelated to the text, which focuses on back pain rather than ankle pain. It introduces a different context that does not reflect the vicious circle of muscle tension and immobility discussed in the passage.