The function of sieve-tube elements is to
a. transport water upward
b. transport carbon dioxide into leaves
c. transport sugars throughout the plant
d. transport water downward submit

option c

Sieve tubes element are use to transport sugars through out the plants.

Explanation:

Sieve tubes element are elongated and specialised living cells found in the phloem of a flowetlring plant.

They help the phloem to function very well.

Their main function is to transport carbohydrates especially sucrose in the plants. E.g transporting of sucrose from leaves to the fruits.

the dark variety, as they are not visible on the dark tree barks

explanation:

the solute can move "downhill," from regions of higher to lower concentration, relying on the specificity of the protein carrier to pass through the membrane. this process is called passive transport or facilitated diffusion, and does not require energy.