21. when did the precambrian era end?
approximately 5.4 million years ago
approximately 2 million years ago
approximately 100,000 years ago
approximately 540 million years ago

The Precambrian era ends abot 540 million years ago.

Explanation:

The Precambrian era is the first geologic era of the Earth, it initiates from the Earth's formation (4600 million years ago) and ends at the beginning of the Cambrian era (541 million years ago). It is the longest geologic era of the Earth.  

due to the fact that it skipped a generation we can tell it is definitely not a dominant trait, therefore it must be recessive. i think this is right let me know!

explanation:

they deliver oxygen to the blood