Is a certification is a official document starting a level of acheivement

A certificate of achievement is a recognition of progress or completion given out by a variety of institutions, including employers, primary schools, secondary schools and institutions of higher learning.

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Explanation:

the density(ratio between mass and volume as m/v) decreases from top to bottom in periodic table group as the size increases and nuclear charge decreases. but in periods density increases from left to right as atom size decreases and nuclear charge increases.

all eukaryotic cell have mitochondria : )