Positive-feedback control systems:

A. a membrane that lines body cavities: While membranes are made of tissues, this describes a specific structure (like a serous membrane), not the definition of tissue itself.

B. a group of similar cells that perform a common function: This is the standard biological definition of a tissue, which sits between the cellular and organ levels of organization.

C. a thin sheet of cells embedded in a matrix: This describes specific types of connective or epithelial tissues, but is too narrow to define all tissues.

D. the most complex organizational unit of the body: This is incorrect; the "organism" is the most complex unit, followed by organ systems and then organs.

A. positive: The outside of the neuron is more positive due to a higher concentration of sodium ions (Na+).

B. negative: During the resting membrane potential, the inside of the neuron is negatively charged (typically -70mV) relative to the outside, largely due to internal proteins and the leakage of potassium.

C. equal: If the charge were equal, there would be no membrane potential, and the neuron could not fire an impulse.

D. None of the above: Incorrect, as choice B is the standard physiological fact.