<span>Pond organism is unicellular and crab is multi cellular.
The simplest level at which life exist is called the cell. Unicellular organism contain only one cell while multi cellular organisms contain more than one cell. Multi cellular organisms can be organized into various levels such as Cells>tissues> organs>organ systems> organism. Thus pond organism is unicellular under microscope and crab is multi cellular.</span>
Oviparity is a mode of reproduction in which young, developed in fertilized eggs, laid outside the mother's body. This mode of reproduction is found in most reptiles and all birds.
Viviparity means that their young develop inside the mother's body and are born alive. This is the mode of most mammals.
<span>Ovoviviparity is similar to viviparity in that there is internal fertilization and the young ones are born alive but differ in that there is no placental connection, the young are fed by egg yolk. This is found i</span><span>n </span><span>seahorses, for example.</span>
Pretty sure its truth / no edit here
"(1) Human population growth reached 6.8 billion in 2010 and it continues to increase" is what led to the other <span>three, since all of these happen from human activity. </span>