`#Sad`
`a)`
`2H_2+O_2` $\xrightarrow{{ t^o}}$ `2H_2O`
`Ca+2H_2O` $\longrightarrow$ `Ca(OH)_2+H_2`
`Ca(OH)_2+2HCl` $\longrightarrow$ `CaCl_2+2H_2O`
`b)`
`2Cu+O_2` $\xrightarrow{{ t^o}}$ `2CuO`
`CuO+2HNO_3` $\longrightarrow$ `Cu(NO_3)_2+H_2O`
`Cu(NO_3)_2+2NaOH` $\longrightarrow$ `Cu(OH)_2+2NaNO_3`
`Cu(OH)_2` $\xrightarrow{{ t^o}}$ `CuO+H_2O`