Interpreting solution of a system of linear equations

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Hi I am solving the following system:

var('C_at C_au C_bt C_bu I J k_1 k_2 gamma_1 gamma_2')
ss = solve([I-k_1*C_at,k_1*C_at - gamma_1*C_au, k_2*C_bu - gamma_2*C_bt, k_2*C_bu -   gamma_2*C_bt],C_at,C_au,C_bt,C_bu)
ss

and I get the following solution:

[[C_at=I/k_1,C_au=I/?_1,C_bt=k_2*r8/?_2,C_bu=r8]]

what is r8?