How muscles are able to contract and relax to bring about movements?

There are small individual fibres in the muscles, known as musclular fibres or muscle fibers that iat the same time are formed by myofibrils.

The most common and accepted theory of how muscles contract is the sliding filament theory:

The theory says that neurons send signals to the muscle fibres, and, inside these, actin and myosin bind with each-other, contracting themselves using ATP as an energy source. That is a simple way of describing how muscle contraction occurs.

The image down below may help you understand the sliding filament concept a bit better.

Hope it helped,

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answer; a male inherits only one allele of the x-linked gene controlling hair color;

i think a) betsy can donate only blood to mark is correct because, mark and betsy have the same blood type. whereas nobody else has b+ blood so betsy can't donate to them.