Read the following passage and mark the letter A, B, C or D on your answer sheet to indicate the correct answer to each of the question 33 to 42.Artificial flowers are used for scientific as well as for decorative purposes. They are made from a variety of materials, such as wax and glass, so skillfully that they can scarcely be distinguished from natural flowers. In making such models, painstaking skill and artistry are called for, as well as thorough knowledge of plant structure. The collection ...

Read the following passage and mark the letter A, B, C or D on your answer sheet to indicate the correct answer to each of the question 33 to 42.

Artificial flowers are used for scientific as well as for decorative purposes. They are made from a variety of materials, such as wax and glass, so skillfully that they can scarcely be distinguished from natural flowers. In making such models, painstaking skill and artistry are called for, as well as thorough knowledge of plant structure. The collection of glass flowers in the Botanical Museum of Harvard University is the most famous in the North America and is widely known throughout the scientific world. In all, there are several thousand models in colored glass, the work of two artist-naturalist, Leopold Blaschka and his son Rudolph.

The intention was to have the collection represent at least one member of each flower family native to the United States. Although it was never completed, it contains more than seven hundred species representing 164 families of flowering plants, a group of fruits showing the effect of fungus diseases, and thousands of flowers parts and magnified details. Every detail of these is accurately reproduced in color and structure. The models are kept in locked cases as they are too valuable and fragile for classroom use.

The word fragile in bold in paragraph 2 most probably means __________.