Consider This Structure Which Bond Represents an Sp-Sp² Carbon-Carbon Bond

Short Answer

Consider this structure:
$$\mathrm { H } _ { 3 } \mathrm { C } \stackrel { a } { - } \mathrm { C } \stackrel { b } { \equiv } \mathrm { C } \stackrel { c } { - } \mathrm { CH } \stackrel { d } { = } \mathrm { CH } \stackrel { e } { - } \mathrm { CH } _ { 2 } \stackrel { f } { - } \mathrm { CH } _ { 3 }$$ Which bond represents an sp-sp² carbon-carbon bond?