Question

what type of bond holds the two strands of dna together?
ionic bond
covalent bond
hydrogen bond
metallic bond
which structure regulates the passage of materials into and out of a cell?
cell wall
cytoplasm
nucleus
cell membrane
which of the following is not part of human circulatory system?
heart
blood vessels
lymph nodes
blood

Explanation:

• For the first question: DNA has two strands held together by hydrogen bonds between complementary base pairs (adenine - thymine and cytosine - guanine). Ionic bonds involve transfer of electrons between ions, covalent bonds are sharing of electrons within a molecule (like in the sugar - phosphate backbone of DNA), and metallic bonds are in metals.
• For the second question: The cell membrane is a selectively permeable structure that regulates what enters and exits the cell. The cell wall (in plant cells) provides structural support, cytoplasm is the fluid inside the cell where organelles are suspended, and the nucleus contains genetic material.
• For the third question: The human circulatory system consists of the heart (pumps blood), blood vessels (transport blood), and blood (carries substances). Lymph nodes are part of the lymphatic system which is related but distinct (it helps in immune function and fluid balance in a different way than the circulatory system's main role of transporting oxygen, nutrients etc. via blood).

Answer:

1. Hydrogen bond
2. Cell membrane
3. Lymph nodes