[单选题]
In the Atwood machine s,l laplr4;j) )ec,l* qarghl4hown fu1q9x6 ticyd //4es yin Fig. 4-1, if M = 0.60 kg and m = 0.40 kg, what is the tensionfq6ui4/9y / ey1tscx d in the string? (Ignore friction and the mass of the pulley.)
A. $3.1 \; N$
B. $4.7 \; N$
C. $7.1 \; N$
D. $7.5 \; N$