stationary(=writing materials) is common collective nouns,not in eg.
uncountable nouns always singular in number,not in eg.
some nouns have no plural forms although they are not material nouns,eg:-baggage, furniture, luggage, rubbish, bread.,not in eg.
B:- There are ( an eraser and a ruler) on the desk.
按文法規則There + be的be與後面的主詞在數上要一致,但在口語時,there is常被當成固定的形式在使用。
There are an eraser and a ruler on the desk.
=An eraser and a ruler are there on the desk.
主詞是1+1 = 2(複數)
所以按文法要用there are ....