Four Journal Entries Are Presented Below $\begin{array}{llr} \text {Cash } &12,200\\ \text { Service Revenue } & &12,200\\\end{array}$

Four journal entries are presented below.Write an explanation for each entry.
A)
$\begin{array}{llr} \text {Cash } &12,200\\ \text { Service Revenue } & &12,200\\\end{array}$

B)
$\begin{array}{llr} \text {Accounts Receivable } &17,500\\ \text {Service Revenue } &&17,500\\\end{array}$

C)
$\begin{array}{llr} \text { Cash } &16,900\\ \text { Accounts Receivable } &&16,900\\\end{array}$

D)
$\begin{array}{llr} \text {Cash } &1,830\\ \text { Unearned Revenue } &&1,830\\\end{array}$