catabolite repression exists in e. coli because: a. it prevents utilizing complex food sources in the presence of simpler sources like glucose b. it represses protein expression globally until sufficient energy in the form of glucose becomes available c. it prevents an overabundance of camp when the cell requires atp d. it allows utilization of glucose at an equal rate to other food sources in order to conserve glucose e. it allows cells to carefully save glucose until all other food sources are exhausted