Consider the DataFrame

df $=$ pd$.$DataFrame$([[$i$-$j$+3$ for i in range$($j$+1)]$ for j in range$(6)])$

Which of the following will return a DataFrame where no entries are null valued.

df$.$fillna$($method$=$'ffill'$)$

df$.$fillna$($method$=$'bfill'$)$

df$.$fillna$($method$=$'ffill', axis $=1)$

df$.$fillna$($method$=$'bfill', axis $=1)$