[单选题]
A block of metal weighs 40 N in air and 30 N in water. 7pm*7 b*rp axsr0ufgxb581rkx 3 qfp( (i oe/w+ rd.bkl 5fc:p2e9m2xfx:tok What is the buoyant force of thqbxepmk+l t2d/9ixpo3: e2k( oc5.frfw (f :xe water?
A. $10 \; N$
B. $30 \; N$
C. $40 \; N$
D. $70 \; N$