Analytical laboratories are often subjected to a quality check in which they have to analyse certain reference samples. In the plot below, the concentrations of lead in a set of reference samples are plotted on the x-axis, and the results from the lab on the y-axis. The slope of the fitted line is 1, and the intercept is 3.4 mg/l. Which statement is correct?

Because the slope of the line is 1, the laboratory results are correct.
The laboratory results are subject to a large random error because the intercept is not equal to zero.
The results of the laboratory are incorrect for low concentrations but OK for high concentrations.
The laboratory makes a systematic error: all concentrations are predicted too high.