Scientists estimate that the earth formed some 4.6 billion years ago with the first forms of life appearing around 3.5 billion years ago. these first forms of life were simple prokaryotes. later, the first photosynethic organisms resulted in more in the atmosphere. this allowed for more higher level organisms to appear on earth. a) carbon b) nitrogen c) oxygen d) sulfur

Which of these statements best describes the st. lawrence seaway? the st. lawrence seaway connects the st. lawrence river with the arctic ocean. the st. lawrence seaway provides an important trading route between the u.s. and mexico. the st. lawrence seaway provides an important trading route for the u.s, but has little value for canada. the st. lawrence seaway connects the great lakes, the st. lawrence river, and the atlantic ocean.

Control of the body is accomplished by which of the following body systems? nervous system and circulatory system endocrine and repertory system circulatory and respiratory systems nervous system and endocrine systems

Choose the correct words to complete the sentences related to genetic screening. is a procedure that is used during pregnancy to detect genetic defects. is extracted from the uterus and used to identify genetic disorders?