B.
[tex]a = 1,2\cdot10^{5}\\\\\\a\cdot10^{3} =1,2\cdot10^{5}\cdot10^{3} = 1,2\cdot10^{5+3} = 1,2\cdot10^{8}\\\\a:10^{3} = 1,2\cdot10^{5}:10^{3} = 1,2\cdot10^{5-3} = 1,2\cdot10^{2}\\\\2a = 2\cdot1,2\cdot10^{5} =2,4\cdot10^{5}\\\\20a = 20\cdot1,2\cdot10^{5} = 2\cdot10\cdot1,2\cdot10^{5} = 2,4\cdot10^{1+5} = 2,4\cdot10^{6}\\\\a^{2} = (1,2\cdot10^{5})^{2} = 1,2^{2}\cdot(10^{5})^{2} = 1,44\cdot10^{5\cdot2} = 1,44\cdot10^{10}[/tex]
C.
[tex]\sqrt{18} = \sqrt{9\cdot2} = \sqrt{9}\cdot\sqrt{2} = 3\sqrt{2}\\\\\sqrt{75} = \sqrt{25\cdot3} = \sqrt{25}\cdot\sqrt{3} = 5\sqrt{3}\\\\\sqrt{172} = \sqrt{4\cdot43}=\sqrt{4}\cdot\sqrt{43} = 2\sqrt{43}\\\\\sqrt[3]{24} = \sqrt[3]{8\cdot3} = \sqrt[3]{8}\cdot\sqrt{3} = 2\sqrt[3]{3}\\\\\sqrt[3]{-128} = \sqrt[3]{-64\cdot2} = \sqrt[3]{-64}\cdot\sqrt[3]{2} = -4\sqrt[3]{2}[/tex]
