For items $27-28$ :

Below is data collected for the analysis of $Ca$ in a supplement tablet using atomic absorption spectrometry. Absorbances were measured for standards, the sample and two sample solutions to which standard Ca was added to increase the Ca concentration

$27.$ Why is standard addition necessary to get an accurate measurement of the $Ca?$

$($A$)$ Both eurves are linear indicating that there is a matrix effect.

$($B$)$ The standard addition plot shows that adding standard to the sample causes an increase in the absorbance$.$

$($C$)$ The intercept of the standard addition plot is significantly larger than the intercept of the calibration curve.

$($D$)$ The standard addition slope is less than the calibration curve slope indicating that there is interference in the sample.

$28.$ What type of interference does a standard addition method help prevent?

$($A$)$ chemical interference that reduces the absorption of light by $Ca$ atoms

$($B$)$ signal interference caused by lamp drift

$($C$)$ spectral interference that requires Zeeman background correction

$($D$)$ spectral interference that adds to the absorption of light by flame components