Assume that a point mutation changes the codon AUU to AUC. Why is this a silent mutation?

It's a silent mutation because it doesn't change the amino acid or the rest of the protein formed by the codons.

before, trains worked using the steam engine. thus, it uses heat so it is crucial to apply heat balance. the steam engine absorbs energy during the stage of boiling water. so, this is the endothermic reaction. when the steam is used to work the piston, it has to release this energy through running the train. the excess energy is released as an exhaust to the chimney. this is the exothermic process because it releases energy to the surroundings.