As can be seen from the figure, the gravity of the metal block is: G=F =54N, so A is wrong and does not meet the meaning of the question.
When the metal block does not touch the liquid level, it is the AB section in the figure.
As can be seen from the figure, when the Zhang Liwei of the rope is 34N, the buoyancy of the metal block is as follows:
F F float =G-F pull = 54 N-34 N = 20 N n. Therefore, statement B is correct and in line with the meaning of the question.
∫F float =ρ water v row g,
∴ Volume of boiling water discharged from metal block (volume of metal block):
V gold =V row =F floating ρ water G = 20N/kloc-0 /×103kg/m3×10n/kg = 0.002m3,
∫G = mg，
∴ The mass of the metal block is m=Gg=54N 10N/kg=5.4kg,
The density of the metal block is ρ gold = MV = 5.4kg0.002m3 = 2.7×103kg/m3. Therefore, D's statement is wrong and does not meet the meaning of the question.
As can be seen from the figure, the tension of the rope gradually increases from t 1 to t2, as can be seen from the formula F float =G-F pull.
The gravity of the metal block remains unchanged, but the tension gradually increases, so the buoyancy gradually decreases.
So choose B.