Which of the following is used to determine the appropriate stringency (Wallace rule)?

• Tm = 2 x (No.of G:C base pair) + 4 x (No.of A:T base pair)
• Tm = 4 x (No.of A:T base pair) + 2 x (No.of G:C base pair)
• Tm = 4 x (No.of G:C base pair) + 2 x (No.of A:T base pair)
• Tm = 69.3° + 0.41 (%)G+C -650/1