Which combination forms a covalent bond? A. Two carbon atoms
B. Two oxygen molecules
C. Two water molecules
D. Two sodium atoms
SUSMIT
!!!

A. Two carbon atoms

Explanation:

We know that covalent bond occurs when there are two non - metallic atoms and carbon atoms are non-metallic so it is the answer.

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the oldest known insects are believed to live around 400 millions ago as suggested by the fossil records.   some plants having woody stems also evolved during this time.

after the evolution of insects the first four legged species or the tetrapods evolved in shallow freshwater habitats from the intermediate species approximately 397 million years ago. the tetrapods went on conquering the land giving rise to amphibians, reptiles, mammals and birds.

the major split in tetrapods first occurred 340 million years ago branching off from amphibians.

363 million years ago when the carboniferous period began the insects started roaming on land and taking flights, following which the four limbed tetrapods adapted in the terrestrial habitat.