Parasitism is when one organism benefits at the cost of it's host. Like fleas on a dog or intestinal parasites to livestock. Fleas and intestinal worms get food and a nice place to livr, while the dogs and the livestock are being leeched off of and malnourished.
Commensalism is when one organism benefits, while the host is left unharmed- but also doesn't really get anything in return, unlike mutualism. Like a shark and remoras (those fish with suction-cup heads & hitchhike). Sharks aren't hurt, but the remoras get free transportation and access to food scraps.
Hope this helps!
The correct answer would be letter B, A covalent bond also known as, molecular bond is a chemical bond sharing two electrons or sharing bonds between atoms.
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All plasmids we manufactured are free of animal-derived materials and can contain low levels of endotoxin (<100EU/mg, on request). This process is designed to meet our customers’ diverse downstream applications such as transfection, antibody preparation, vaccine, and gene-therapy research, etc.