$\text{Câu 5:}$
$\text{Ta có:}$
`8m^2 3dm^2 = ... dm^2`
`8 m^2 = 800 dm^2`
$\Rightarrow$ `8m^2 3dm^2 = 800 dm^{2} + 3 dm^{2} = 803 dm^2`
$\Rightarrow$ $\text{Chọn D}$
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$\text{Câu 6:}$
`12 km 4 m = ... m`
`12 km = 12000 m`
$\Rightarrow$ `12 km 4m = 12 000 m + 4 m = 12004 m`
$\Rightarrow$ $\text{Chọn C}$