Mass-loss history of an 18 M_⊙ donor star undergoing binary interaction. The left panel shows the Case B model, and the right panel shows the Case C model. The companion mass is 16.2 M_⊙, with initial orbital separations of 2400 R_⊙ (Case B) and 3000 R_⊙ (Case C). The x-axis indicates time before core collapse, and the y-axis shows the mass-loss rate. Although Case B can reach high mass-loss rates, this occurs ∼1 Myr before core collapse and therefore does not contribute to the formation of dense CSM immediately prior to explosion. In contrast, Case C mass transfer occurs ∼10⁴ yr before core collapse and can persist until explosion.