The inhibitory IкBα protein binds to the NFкB dimer and holds them in an inactive state. On TNF treatment, the IкBα protein is degraded and NFкB enters the nucleus to transactivate gene expression. Importantly, one of the transcriptional targets of NFкB is the IкBα gene itself. Together, they form a negative feedback loop. Which one of the following graphs represents the NFKB protein expression kinetics when TNF exposure is given as a pulse (a) or continuously (b)?