Прочитайте текст и заполните пропуски подходящими по смыслу словами
Intensive 1______________ of radio-location began in the early forties. All these years the vacuum 2_______________ was the heart of electronics and, hence, the field of 3______________ was usually defined as the study of the motion of electrons in vacuum tubes. However, it was found that small, controlled 4______________ of gas introduced into a "vacuum tube" could make it perform switchings, rectifying and other jobs. To include the category of gas-filled tubes, "vacuum tubes" became converted to "electron tubes" and the science of electronics was expanded to include the study of electron motion in gases.
development
tube
electronics
quantities