Consider the following schedules involving two transactions.
$S_1 : R_1(x), W_2(y),R_2(x),W_1(y), \ commit_1, \ commit_2$
$S_2 : R_1(x), R_2(y),W_1(z), \ commit_1,R_3(y), R_3(z), W_2(y), W_3(x), commit_2, commit_3$
Which of the following statements is true?
A) Both S1 and S2 are allowed under 2PL
B) only S1 is allowed under 2PL
C) only S2 is allowed under 2PL
D) neither S1 nor S2 is allowed under 2PL