5. Read and answer. Dmitri Mendeleev was a Russian scientist who developed the periodic table of elements. Before he presented it in 1869, he had been studying the properties of elements for a long time. His work helped scientists develop chemical knowledge. a) What did Mendeleev develop? b) What had he been studying before he presented the table? c) Find the Past Perfect Continuous in the text.

Перевод задания: Прочитайте текст и ответьте на вопросы.

1. He developed the periodic table of elements.
   Перевод: Он разработал периодическую таблицу химических элементов.
2. He had been studying the properties of elements.
   Перевод: Он изучал свойства химических элементов.
3. had been studying — форма Past Perfect Continuous.
   Перевод: «изучал».

Ответ: a) The periodic table of elements. b) The properties of elements. c) had been studying.