Select the rows where the age is missing (NaN).

Input

  animal  age  visits priority
a    cat  2.5       1      yes
b    cat  3.0       3      yes
c  snake  0.5       2       no
d    dog  NaN       3      yes
e    dog  5.0       2       no
f    cat  2.0       3       no
g  snake  4.5       1       no
h    cat  NaN       1      yes
i    dog  7.0       2       no
j    dog  3.0       1       no

Output

  animal  age  visits priority
d    dog  NaN       3      yes
h    cat  NaN       1      yes