- AActinium and Thorium.
- BActinium and Nobelium.
- CActinium and Mendelevium.
- DActinium and Lawrencium.
Explanation:
We have 15 elements in actinides. They are from atomic number 89 to 103 Actinium, Thorium, Protactinium, Uranium, Neptunium, Plutonium, Americium, Curium, Berkelium, Californium, Einsteinium, Fermium, Mendelevium, Nobelium, Lawrencium. Hence, from these list we could say that Actinium is the first and Lawrencium is the last element in actinides.
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$x=$.. . . . . .(nearest integer)
[Given: Molal freezing point depression constant of water at $273.15 \mathrm{~K}^2$ is $1.86 \mathrm{~K} \mathrm{~kg} \mathrm{~mol}^{-1}$ ]

$(A)\,{{C}_{8}}{{H}_{10}}\xrightarrow{KMn{{O}_{4}}}(B){{C}_{8}}{{H}_{6}}{{O}_{4}}\xrightarrow[Fe]{B{{r}_{2}}}{{C}_{8}}{{H}_{5}}Br{{O}_{4}}(C)$ (one-product only)
$X\xrightarrow[\Delta ]{{{H}_{2}}O}A+B+C+D$