In the following, when a transaction T performs an operation x$=$x$+$a$,$ then T reads x into some register, adds a to the register content then stores register content into x$.$

A transaction schedule is serializable if its effect is equivalent to that of some serial schedule. Consider a bookkeeping operation consisting of two transactions $$T$1$ and T$2$ that are required to keep the sum A$+$B$+$C unchanged. Which of the following pairs of transactions will always result in a serializable schedule?

I only

II only

III only

I and II