Chemical B, which floods the muscle cells' cytoplasm with Ca2+, would not be a good choice for a muscle relaxer. In order to determine this, one must understand the function of Ca2+,and calcium, in general, within muscle cells. Calcium is responsible for aiding in the process of muscle movement and blood circulation throughout your body. Specifically, Ca2+ is a calcium ion responsible for contraction of the skeletal muscles.
On the cellular level, the calcium ions are released from the sarcoplasmic reticulum and attach themselves to a molecule of troponin. Through this attachment, tropomyosin is displaced from actin filaments. Tropomyosin is a protein that regulates the contraction of skeletal muscles. Myosin is a protein filament that attaches to actin filaments and tugs on it, causing contraction of the muscle through the actin-myosin complex. When Ca2+ is released from the sarcoplasmic reticulum and binds to the troponin, the displacement of the tropomyosin occurs, causing exposure of the actin filaments, which the myosin then attach itself to, and thus, the actin-myosin complex is activated and skeletal muscle contraction occurs.
As such, flooding the muscle cell with calcium ions would not serve to relax the muscles; instead, it would activate the actin-myosin complex through the displacement of the tropomyosin, and skeletal muscle movement would occur.