Please help with $2$a$-2$d$.$ Im not sure if $2$a is correct or not.

Three calibration curves are shown $($A$,$ B$,$ C$,$ and D$)$ on the next page and the results of linear regression analyses are included on each graph. One curve is an external calibration curve, one curve is a standard addition curve, one curve is an internal standard curve. A fourth curve is included as well. The signal obtained for the unknown sample is plotted as well and the response is written near the linear regression equation.

$2$a$.$

Which of the four calibration curves is an external calibration curve?

Using the linear regression and the signal for the unknown sample, calculate the concentration of the unknown. Report this calculated concentration of the unknown sample to $3$ significant figures.

Do NOT attempt to calculate the error associated with the unknown.

$y=mx+b,x=\frac{(0\mathrm{.}035)}{24\mathrm{.}521}$

$0=24\mathrm{.}521x-0\mathrm{.}035,=\mathrm{.}00144$

$0\mathrm{.}035=24\mathrm{.}521x,$

$2b$

Which of the four calibration curves is a standard addition calibration curve?

Using the linear regression and the signal for the unknown sample, calculate the concentration of the unknown. Report this calculated concentration of the unknown sample to $3$ significant figures.

Do NOT attempt to calculate the error associated with the unknown.

$2$c

Which of the four calibration curves is an internal standard addition calibration curve?

Using the linear regression and the signal for the unknown sample, calculate the concentration of the unknown. Report this calculated concentration of the unknown sample to $3$ significant figures.

Do NOT attempt to calculate the error associated with the unknown.

$2$d$.$

Explain why the remaining calibration curve is not suitable a choice. Justify your answer!

external

C $y=454\mathrm{.}88x+9\mathrm{.}2481R^2=0\mathrm{.}998,$ determined from response ratio for standards $1-5$ cirnal fnr camnla $=21nd$

Internal

R $y=0\mathrm{.}0088x+0\mathrm{.}2022R^2=0\mathrm{.}999,$ determined from standards $1-5$

D $y={19\mathrm{.}0}_{6}x+{0\mathrm{.}0356}_5{R}^2=0\mathrm{.}9998,$ determined using standards $1-5$

standard

addition