[单选题]
Cloud chambers have beea*t xj/g dpk)7n replaced by bubble chambers beca6- u ej)dx+phbuse
A. the radioactive clouds were too dangerous to work with.
B. the density of fluids is greater than the density of vapors.
C. bubble chambers tend to be larger and more expensive.
D. gamma rays are visible in bubble chambers, but not in cloud chambers.
