Which layer consists on average of 25 to 30 layers of flattened dead keratinocytes. stratum basale stratum spinosum stratum granulosum stratum lucidum stratum corneum

b. did not initially have a mutually beneficial relationship with their hosts, but later evolved one

Explanation:

Initially, the host cell engulfed bacteria as food material. It did not have a mutually beneficial relationship with it. However, many engulfed bacteria had several advantages which could also benefit the host cell. For example, some of them could synthesis food material by the process of photosynthesis. The host cell then formed symbiotic relationship with these bacteria. They survived inside the host cell and in return they also supplied the host with prepared food material. Eventually this paved the way for endosymbiosis and origin of many organelles like chloroplast and mitochondria.