1. Right. GAPDH is a house keeping gene. Thus it is expressed in different cells at equal amounts. For that reason we (and others) choose to use it as a control in Northern blot experiments.  You may notice that the expression level of GAPDH in the brain is slightly reduced when compared to those of the heart and liver. However this difference may result from slight inaccuracies during sample loading and should not be attributed to actual differences between the different tissues.   Continue.

2. Wrong. That is true for the GOI expression profile but not for GAPDH.

  Try again.

3. Right. You are very perceptive and notice even small differences. However, this difference may result from slight inaccuracies during sample loading and should not be attributed to actual differences between the different tissues.
GAPDH is a house keeping gene. Thus it is expressed in different cells at equal amounts. For that reason we (and others) choose to use it as a control in Northern blot experiments.