The interaction between an antigen $(Ag)$ and a single-chain antibody $(Ab)$ was studied using Scatchard analysis. The result is shown below.

The affinity of interaction and the total concentration of antibody, respectively, can be determined from

1. slope and $Y$-intercept
2. $Y$-intercept and slope
3. $X$-intercept and slope
4. slope and $X$-intercept