A.Funds for research were insufficient
B.The source of illumination was not bright enough until recently
C.Materials used to manufacture X-ray tubes were difficult to obtain
D.X-ray microscopes were too complicated to operate

A.To explain how it functioned
B.To show how limited its uses are
C.To begin a discussion of sixteenth century discoveries
D.To put the X-ray microscope in historical perspective

A.complex
B.tiny
C.circular
D.dangerous

A.understand more about the distribution of the chemical elements
B.develop the electron microscope later on
C.discover single-celled plants and animals they had never seen before
D.see viruses directly

A.Sources of illumination for microscopes
B.The detail seen through a microscope
C.Outdated microscopic technique
D.A new kind of microscope

A.Sources of illumination for microscopes
B.The detail seen through a microscope
C.Outdated microscopic technique
D.A new kind of microscope

A.that
B.what
C.which
D.how

A.as soon as
B.provided
C.so that
D.already

A.surpassed
B.overtaken
C.exceeded
D.outdone

A.example
B.understanding
C.clarification
D.explanation