Read the following sentence and use context clues to define the underlined word: It was hard for the detective to solve the killer's abstract riddle.
a. being present as an idea but not in a definite form
b. being obscure as an idea and a definite form
c. being present as an idea and a definite form
d. not being present as an idea but in a definite form

Walking is not the most exciting form of exercise a person can take on for fitness. yet it is low impact and requires no fancy equipment. that makes walking a simple activity for those new to exercise. walking is less likely than other exercises to cause injury to leg tendons and muscles. all it requires is a good pair of sneakers-no cables, stylish outfits, or weights required. of course, a person could increase the benefit of a workout by adding weights or speed walking. identify the main idea of this paragraph. walking is a difficult and safe exercise for beginners. walking is an easy and safe exercise for beginners. walking is a boring and dangerous form of exercise. walking is an exciting and very effective form of exercise.