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C.Outdated microscopic technique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C.Outdated microscopic technique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

A.lost
B.missing
C.absent
D.misplaced

A.lost
B.missing
C.absent
D.misplaced

A.retreat
B.conquer
C.gather
D.remain