<em>The correct option is cell surface markers.</em>
Explanation:
The immune cells of our body detect foreign particles and generate responses so that our body can get rid of them. The foreign particles are often termed as antigens.
The immune cells such as antibodies possess cell surface receptors which detect the foreign objects or antigens. When the cell surface receptors detect any antigen they immediately recognize that a foreign particle has invaded the body and they then identify it and start to generate response.
Newtonian gravity is what scientists believe will stop the universe’s expansion. This attraction would have to overcome the propulsive force of dark energy expansion, thus causing the compression of all the matter in the universe back into a small, hyperdense singularity.
